Semiconductor device and method for manufacturing the same

ABSTRACT

To give stable electrical characteristics to and improve reliability of a semiconductor device including a transistor in which an oxide semiconductor film is used for a channel formation region. As a base film, an insulating film or an oxide semiconductor film is used. A single-layer metal film is formed over the base film. After that, a resist mask is formed, and etching is performed plural times. Accordingly, electrodes each including projecting portions when seen in cross-section are formed. Even when a gate insulating film over the source electrode layer and the drain electrode layer or an oxide semiconductor film has a small thickness, disconnection of the gate insulating film is unlikely to occur.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including an oxide semiconductor and a manufacturing method thereof.

In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and an electronic device are all semiconductor devices.

2. Description of the Related Art

A technique for manufacturing a transistor or the like by using an oxide semiconductor film for a channel formation region and applying it to a display device has been attracting attention. Examples of such a transistor include a transistor in which zinc oxide (ZnO) is used as an oxide semiconductor film and a transistor in which InGaO₃(ZnO)_(m) is used as an oxide semiconductor film. A technique for manufacturing such a transistor including an oxide semiconductor film over a light-transmitting substrate and applying it to a switching element or the like of an image display device is disclosed in Patent Documents 1 and 2.

Patent document 3 discloses a structure of a transistor including a source electrode and a drain electrode in contact with an oxide semiconductor layer, a gate electrode overlapping with the oxide semiconductor layer, and a gate insulating film provided between the oxide semiconductor layer and the gate electrode, in which the source electrode and the drain electrode each include a first conductive layer, and a second conductive layer having a region which extends in a channel length direction from an end portion of the first conductive layer.

In Patent Document 4, a structure in which gate electrodes are formed below and over an oxide semiconductor layer is disclosed.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.     2007-123861 -   [Patent Document 2] Japanese Published Patent Application No.     2007-096055 -   [Patent Document 3] Japanese Published Patent Application No.     2007-171721 -   [Patent Document 4] Japanese Published Patent Application No.     2011-103458

SUMMARY OF THE INVENTION

One object of the present invention is to provide a structure of a transistor in which a source electrode layer and a drain electrode layer are formed using a single-layer metal film over an oxide semiconductor film and electric-field concentration which might occur in end portions of the source electrode layer and the drain electrode layer is reduced.

Another object of the present invention is to give stable electrical characteristics to a semiconductor device including a transistor in which an oxide semiconductor film is used for a channel formation region and to improve reliability of the semiconductor device.

Another object of the present invention is to form a source electrode layer and a drain electrode layer each having a cross-sectional shape with which disconnection of a gate insulating film is unlikely to occur even when the gate insulating film over the source electrode layer and the drain electrode layer or an oxide semiconductor film has a small thickness.

As a base film, an insulating film or an oxide semiconductor film is used. A single-layer metal film is formed over the base film. After that, a resist mask is formed, and etching is performed plural times. Accordingly, electrodes each including projecting portions when seen in cross-section are formed.

Specifically, a resist mask is formed over a metal film, first etching for removing the metal film to approximately half the thickness so that the metal film partly has a small thickness is performed, and ashing is performed on the resist mask for reducing the area of the resist mask. After that, second etching is performed with the resist mask, which is reduced in size, so that single-layer metal films each including projecting portions are formed.

In the case where an oxide semiconductor film is used as the base film, when or after a single-layer metal film in contact with the oxide semiconductor film is processed, etching may be performed with the use of the metal film as a mask so as to form a thin-film region of the oxide semiconductor film (that is, a region which has a smaller thickness than a region overlapping with the metal film and does not overlap with the metal film). In such a case, a transistor in which the thin-film region serves as a channel formation region can be manufactured.

In the case where a source electrode layer and a drain electrode layer are formed over the oxide semiconductor film, the source electrode layer and the drain electrode layer each including projecting portions extending in the channel length direction can be formed by the above steps. In the case where a gate insulating film is formed over the source electrode layer and the drain electrode layer each having such a cross-sectional shape, disconnection of the gate insulating film is unlikely to occur even when the gate insulating film or the oxide semiconductor film has a small thickness. By forming the source electrode layer and the drain electrode layer each including the projecting portions extending in the channel length direction using a single-layer metal film, the manufacturing process can be simple as compared to a case of using stacked metal films.

Some of the projecting portions of the source electrode layer and the drain electrode layer overlap with the oxide semiconductor film; thus, electric-field concentration is reduced, which contributes to improvement in electrical characteristics and reliability of a transistor.

By providing gate electrode layers below and over an oxide semiconductor film and setting the potential of one of the gate electrode layers to GND, the threshold voltage of the transistor is increased, so that a normally-off transistor can be provided.

In addition, the gate electrode layer having a potential of GND has a function of blocking an external electric field, that is, a function of preventing an external electric field (particularly, a function of preventing static electricity) from affecting a transistor. A blocking function of the gate electrode layer having a potential of GND can prevent variation in electrical characteristics of the transistor due to the effect of an external electric field such as static electricity.

One embodiment of the invention disclosed in this specification is a semiconductor device including a conductive layer; an oxide insulating film over the conductive layer; an oxide semiconductor film which is over the oxide insulating film and includes a channel formation region; a source electrode layer and a drain electrode layer which are over the oxide semiconductor film and are electrically connected to the oxide semiconductor film; a gate insulating film over the oxide semiconductor film, the source electrode layer, and the drain electrode layer; and a gate electrode layer which is over the gate insulating film and overlaps with the channel formation region. The source electrode layer and the drain electrode layer are formed using a single-layer metal film and each include projecting portions extending in the channel length direction. Some of the projecting portions which overlap with the oxide semiconductor film overlap with the conductive layer.

The above structure, which includes the source electrode layer and the drain electrode layer each including the projecting portions extending in the channel length direction, contributes to reduction in electric-field concentration and improvement in electrical characteristics and reliability of a transistor.

Since the source electrode layer and the drain electrode layer each including the projecting portions extending in the channel length direction are included, disconnection of the gate insulating film is unlikely to occur even when the gate insulating film provided over the source electrode layer and the drain electrode layer or the oxide semiconductor film has a small thickness. By forming the source electrode layer and the drain electrode layer each including the projecting portions extending in the channel length direction using a single-layer metal film, the process can be simple.

In the above structure, the conductive layer is provided so as to be embedded in the oxide insulating film. In the oxide insulating film, an oxygen-excess region which contains oxygen whose proportion is higher than that in the stoichiometric composition of the oxide semiconductor film exists in the vicinity of the bottom surface of the oxide insulating film and the vicinity of the conductive layer in a region where the conductive layer exists.

The oxygen-excess region can be formed by forming a conductive layer, forming an oxide insulating film over the conductive layer, and then performing oxygen adding treatment (oxygen doping treatment) on the oxide insulating film having the top surface including a projecting portion reflecting the shape of the conductive layer. After forming the oxygen-excess region, planarization treatment is performed on the oxide insulating film in order to remove the projecting portion of the top surface. For the planarization treatment, a chemical mechanical polishing method is used.

By the planarization treatment, the oxide insulating film over the conductive layer is selectively removed to be thin, whereby the distance between the oxygen-excess region over the conductive layer and the top surface of the oxide insulating film becomes shorter. In contrast, a region of the oxide insulating film in which the conductive layer does not exist is hardly removed, and the oxygen-excess region exists in the vicinity of the bottom surface of the oxide insulating film. Thus, in the oxide insulating film, the oxygen-excess region is provided at a shallow position from the top surface of the oxide insulating film in the region where the conductive layer exists and is provided at a deep position from the top surface of the oxide insulating film in another region (i.e., the region where the conductive layer does not exist).

Since the oxygen-excess region can be provided close to the oxide semiconductor film in a part of the oxide insulating film over which the oxide semiconductor film (at least the channel formation region) is provided and which overlaps with the conductive layer, oxygen can be efficiently supplied from the oxygen-excess region to the oxide semiconductor film. Supply of oxygen can be promoted by heat treatment.

Thus, in the semiconductor device, oxygen vacancies in the oxide semiconductor film and at the interface between the oxide insulating film and the oxide semiconductor film can be compensated efficiently.

As the oxide semiconductor film including the channel formation region, stacked oxide semiconductor films having different compositions can be used. For example, stacked films in which an In—Ga—Zn-based oxide film formed with a target having an atomic ratio of In:Ga:Zn=3:1:2 (=1/2:1/6:1/3) and an In—Ga—Zn-based oxide film formed with a target having an atomic ratio of In:Ga:Zn=1:1:1 (=1/3:1/3:1/3) are stacked in this order are used. In this case, a film in contact with the gate insulating film is preferably the In—Ga—Zn-based oxide film formed with the target having an atomic ratio of In:Ga:Zn=1:1:1.

By performing etching when or after the source electrode layer and the drain electrode layer each including the projecting portions extending in the channel length direction are formed, a depressed portion may be formed in the oxide semiconductor film including the channel formation region. Forming a depressed portion in the oxide semiconductor film leads to reduction in electric-field concentration and can suppress degradation of switching characteristics. In the case where the stacked oxide semiconductor films having different compositions are used, the stacked oxide semiconductor films are preferably formed so that an upper film of the stacked films remains and thus a lower film of the stacked films is not exposed.

In this specification, a channel length L is a distance between a projecting portion which is one of the projecting portions of a source electrode layer extending in the channel length direction and overlaps with a conductive layer and an oxide semiconductor film and a projecting portion which is one of the projecting portions of a drain electrode layer extending in the channel length direction and overlaps with a conductive layer and the oxide semiconductor film (a horizontal distance when seen from the top surface). Thus, a channel length L of a transistor which does not have a depressed portion in an oxide semiconductor film and has a flat surface is considered as the same as a channel length L of a transistor which has a depressed portion in an oxide semiconductor film.

In the above structure, an insulating layer may be further provided so as to overlap with the source electrode layer and the drain electrode layer, and a gate insulating film may be provided over and in contact with the insulating layer. This insulating layer reduces parasitic capacitance formed between the gate electrode layer and the source electrode layer and parasitic capacitance formed between the gate electrode layer and the drain electrode layer. Further, this insulating layer is formed using a silicon oxide film, a silicon oxynitride film, or the like, and protects the source electrode layer and the drain electrode layer when the gate insulating film or the gate electrode layer is etched.

In the above structure, an insulating film containing gallium is used as a gate insulating film. Examples of the insulating film are a gallium oxide film (also represented by GaO_(X): X may be or may not be a natural number), a gallium zinc oxide film (also represented by Ga_(Z)Zn_(X)O_(Y) (X=1 to 5)), a Ga₂O₃ (Gd₂O₃) film, and an insulating In—Ga—Zn-based oxide film which has a low indium content. The insulating film containing gallium preferably contains a lot of oxygen, and thus the formation conditions of the insulating film containing gallium is made so that the insulating film containing gallium contains a lot of oxygen or oxygen doping treatment is performed after the insulating film containing gallium is formed.

Note that the “oxygen doping” means that oxygen (which includes at least one of an oxygen radical, an oxygen atom, an oxygen molecule, ozone, an oxygen ion (oxygen molecular ion), and/or an oxygen cluster ion) is added to a bulk. Note that the term “bulk” is used in order to clarify that oxygen is added not only to a surface of a thin film but also to the inside of the thin film. In addition, “oxygen doping” includes “oxygen plasma doping” in which oxygen which is made to be plasma is added to a bulk.

A gas containing oxygen can be used for oxygen doping treatment. As the gas containing oxygen, oxygen, dinitrogen monoxide, nitrogen dioxide, carbon dioxide, carbon monoxide, and the like can be used. Further, a rare gas may be included in the gas containing oxygen for the oxygen doping treatment.

Depending on the conditions of the oxygen doping treatment, not only a film which is directly exposed to the oxygen doping treatment but also a film which is provided below the film can be doped with oxygen. That is, by forming the gate insulating film which is a gallium oxide film over the oxide semiconductor film and performing oxygen doping treatment, oxygen can be added not only to the gate insulating film but also to the oxide semiconductor film.

One embodiment of the present invention is a semiconductor device in which insulating films containing gallium such as a gallium oxide film, a gallium zinc oxide film, or a Ga₂O₃ (Gd₂O₃) film are provided below and over and in contact with an oxide semiconductor film to sandwich the oxide semiconductor film. The semiconductor device having the structure includes a conductive layer; an oxide insulating film over the conductive layer; a first insulating film containing gallium oxide over the oxide insulating film; an oxide semiconductor film which is over and in contact with the first insulating film and includes a channel formation region; a source electrode layer and a drain electrode layer which are over the oxide semiconductor film and are electrically connected to the oxide semiconductor film; a second insulating film containing gallium oxide over the oxide semiconductor film, the source electrode layer, and the drain electrode layer; and a gate electrode layer which is over the second insulating film and overlaps with the channel formation region.

In the above structure, the source electrode layer and the drain electrode layer are formed using a single-layer metal film. The source electrode layer and the drain electrode layer each include projecting portions extending in the channel length direction. Some of the projecting portions which overlap with the oxide semiconductor film overlap with the conductive layer.

In the above structure, the oxide semiconductor film preferably has a crystal structure. The oxide semiconductor film having a crystal structure is a single crystal film, a microcrystalline film, a polycrystalline film (also referred to as polycrystal), or a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film.

The CAAC-OS film is not completely single crystal nor completely amorphous. The CAAC-OS film is an oxide semiconductor film including crystal parts. Note that in most cases, the crystal part fits inside a cube whose one side is less than 100 nm. In an image obtained with a transmission electron microscope (TEM), a boundary between adjacent crystal parts in the CAAC-OS film is not clearly detected. Further, with the TEM, a grain boundary in the CAAC-OS film is not found. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is suppressed.

In each of the crystal parts included in the CAAC-OS film, c-axes are aligned in the direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, triangular or hexagonal atomic arrangement which is seen from the direction perpendicular to the a-b plane is formed, and metal atoms are arranged in a layered manner or metal atoms and oxygen atoms are arranged in a layered manner when seen from the direction perpendicular to the c-axis. Note that, among crystal parts, the directions of the a-axis and the b-axis of one crystal part may be different from those of another crystal part. In this specification, a simple term “perpendicular” includes a range from 85° to 95°. In addition, a simple term “parallel” includes a range from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarily uniform. For example, in the formation process of the CAAC-OS film, in the case where crystal growth occurs from a surface side of the oxide semiconductor film, the proportion of crystal parts in the vicinity of the surface of the oxide semiconductor film is higher than that in the vicinity of the surface where the oxide semiconductor film is formed in some cases. By adding impurities to the CAAC-OS film, crystallinity of a crystal part in a region to which the impurities are added is reduced, and depending on the conditions for adding impurities, the region becomes a microcrystalline region or an amorphous region in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film are aligned in the direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, the directions of the c-axes may be different from each other depending on the shape of the CAAC-OS film (the cross-sectional shape of the surface where the CAAC-OS film is formed or the cross-sectional shape of the surface of the CAAC-OS film). Note that the direction of c-axis of the crystal portion is the direction parallel to a normal vector of the surface where the CAAC-OS film is formed or a normal vector of the surface of the CAAC-OS film. The crystal portion is formed by deposition or by performing treatment for crystallization such as heat treatment after deposition.

In an oxide semiconductor having a crystal portion such as the CAAC-OS, defects in the bulk can be further reduced and when the surface flatness of the oxide semiconductor is improved, mobility higher than that of an oxide semiconductor in an amorphous state can be obtained. In order to improve the surface flatness, the oxide semiconductor is preferably formed over a flat surface. Specifically, the oxide semiconductor may be formed over a surface with the average surface roughness (Ra) of less than or equal to 1 nm, preferably less than or equal to 0.3 nm, more preferably less than or equal to 0.1 nm.

With the use of the CAAC-OS film in a transistor, change in electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light can be reduced. Thus, by using a CAAC-OS film as the oxide semiconductor film, it is possible to provide a highly reliable semiconductor device in which change in electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light can be suppressed.

In the case where the oxide semiconductor film is a CAAC-OS film, the first insulating film and the second insulating film which are in contact with the CAAC-OS film preferably have a lower crystallinity than the oxide semiconductor film. Specifically, the first insulating film and the second insulating film are formed at a temperature lower than a film formation temperature of the CAAC-OS film or are formed using a rare gas (e.g., argon) as a film formation gas for sputtering.

Oxygen doping treatment may be performed on the first insulating film containing gallium oxide so that the crystallinity is reduced, and an oxygen-excess region which contains oxygen whose proportion is higher than that in the stoichiometric composition of the first insulating film containing gallium oxide may be formed. For example, in the case of a gallium oxide film represented by Ga₂O₃, the oxygen-excess region is GaO_(X) (X is greater than 1.5). The first insulating film including the oxygen-excess region prevents oxygen from being released from the oxide semiconductor film and effectively supplies oxygen to the oxide semiconductor film.

Oxygen doping treatment may be performed on the second insulating film containing gallium oxide so that the crystallinity is reduced, and an oxygen-excess region which contains oxygen whose proportion is higher than that in the stoichiometric composition of the second insulating film containing gallium oxide may be formed. The second insulating film including the oxygen-excess region prevents oxygen from being released from the oxide semiconductor film and effectively supplies oxygen to the oxide semiconductor film.

As the oxide semiconductor film, a semiconductor film containing gallium (e.g., an In—Ga—Zn-based oxide film) is used, and insulating films containing gallium (e.g., a gallium oxide film) are provided below and over and in contact with the oxide semiconductor film to sandwich the oxide semiconductor film. In such a case, the insulating films provided below and over the oxide semiconductor film contain the same constituent material as that of the oxide semiconductor film, and thus the interface state between the oxide semiconductor film and the insulating films can be excellent, which gives stable electrical characteristics. By providing the insulating films, which contain gallium oxide, below and over and in contact with the oxide semiconductor film to sandwich the oxide semiconductor film, the insulating films can prevent impurities such as nitrogen and a metal element, which might affect the oxide semiconductor film, from entering from the outside by diffusion. Thus, by providing the insulating films containing gallium oxide to sandwich or cover the oxide semiconductor film, the composition and the purity of the covered oxide semiconductor film are kept constant, which gives stable electrical characteristics.

The source electrode layer and the drain electrode layer each having a cross-sectional shape with which disconnection of the gate insulating film is unlikely to occur even when the gate insulating film has a thickness less than or equal to 20 nm or the oxide semiconductor film has a thickness less than or equal to 30 nm. Further, the source electrode layer and the drain electrode layer are formed over the oxide semiconductor film using a single-layer metal film, and electric-field concentration which might occur in end portions of the source electrode layer and the drain electrode layer is reduced.

By using insulating films containing gallium oxide which are below and over and in contact with the oxide semiconductor film to sandwich the oxide semiconductor film, electrical characteristics become stable and reliability can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are cross-sectional views illustrating manufacturing steps of one embodiment of the present invention.

FIGS. 2A to 2C are cross-sectional views illustrating manufacturing steps of one embodiment of the present invention.

FIGS. 3A to 3C are cross-sectional views of embodiments of the present invention.

FIGS. 4A to 4E are cross-sectional views illustrating manufacturing steps of one embodiment of the present invention.

FIGS. 5A to 5C are cross-sectional views of embodiments of the present invention.

FIGS. 6A and 6B are a cross-sectional view and a top view illustrating one embodiment of the present invention.

FIGS. 7A and 7B are a cross-sectional view and a circuit diagram illustrating one embodiment of a semiconductor device.

FIGS. 8A to 8C are a cross-sectional view and circuit diagrams illustrating one embodiment of a semiconductor device.

FIG. 9 is a circuit diagram illustrating one embodiment of a semiconductor device.

FIG. 10 is a perspective view illustrating one embodiment of a semiconductor device.

FIGS. 11A to 11C are block diagrams illustrating one embodiment of a semiconductor device.

FIGS. 12A to 12C are drawings illustrating an electronic device.

FIGS. 13A to 13C are drawings illustrating electronic devices.

FIG. 14 is a cross-sectional view illustrating one embodiment of the invention.

FIGS. 15A to 15C are cross-sectional views of embodiments of the present invention and a cross-sectional image of an end portion of an electrode layer.

FIGS. 16A and 16B are cross-sectional images of end portions of electrode layers.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention are described in detail with reference to the accompanying drawings. However, the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways without departing from the spirit and the scope of the present invention. Therefore, the present invention is not construed as being limited to description of the embodiments.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and a method for manufacturing the semiconductor device is described with reference to FIGS. 1A to 1D. In this embodiment, an example of a method for manufacturing a transistor including an oxide semiconductor film is described.

A conductive film is formed over a substrate 400 having an insulating surface by a sputtering method, an evaporation method, or the like. The conductive film is etched so that a conductive layer 491 is formed.

There is no particular limitation on a substrate that can be used as the substrate 400 having an insulating surface as long as it has heat resistance enough to withstand heat treatment performed later. For example, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like, a ceramic substrate, a quartz substrate, or a sapphire substrate can be used. A single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon, carbon silicon, or the like; a compound semiconductor substrate of silicon germanium or the like; an SOI substrate; or the like can be used as the substrate 400, or the substrate provided with a semiconductor element can be used as the substrate 400.

The conductive layer 491 can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, or scandium or an alloy material which contains any of these materials as its main component. Alternatively, a semiconductor film typified by a polycrystalline silicon film doped with an impurity element such as phosphorus, or a silicide film such as a nickel silicide film may be used as the conductive layer 491. The conductive layer 491 may have a single-layer structure or a stacked-layer structure.

The conductive layer 491 can also be formed using a conductive material such as indium oxide-tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium oxide-zinc oxide, or indium tin oxide to which silicon oxide is added. It is also possible that the conductive layer 491 has a stacked structure of the above conductive material and the above metal material.

In order to obtain a normally-off switching element, it is preferable that the threshold voltage of the transistor is made positive by using a material having a work function of 5 eV (electron volts) or higher, preferably 5.5 eV or higher, for a gate electrode layer. Specifically, a material which includes an In—N bond and has a specific resistivity of 1×10¹⁰⁻¹ Ω·cm to 1×10¹⁰⁻⁴ Ω·cm, preferably 5×10¹⁰⁻² Ω·cm to 1×10¹⁰⁻⁴ Ω·cm, is used for the gate electrode layer. Examples of the material are an In—Ga—Zn-based oxide film containing nitrogen, an In—Sn—O film containing nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O film containing nitrogen, an In—O film containing nitrogen, and a metal nitride film (e.g., an InN film).

Next, an oxide insulating film 480 is formed over the substrate 400 and the conductive layer 491 (see FIG. 1A). The oxide insulating film 480 has a projecting portion reflecting the shape of the conductive layer 491 on its surface.

The oxide insulating film 480 can be formed by a plasma CVD method, a sputtering method, or the like, using silicon oxide, silicon oxynitride, aluminum oxide, aluminum oxynitride, hafnium oxide, gallium oxide, gallium oxide zinc, zinc oxide, or a mixed material of any of these materials. The oxide insulating film 480 may have either a single-layer structure or a stacked-layer structure.

A silicon oxide film formed by a sputtering method is used as the oxide insulating film 480 in this embodiment. Alternatively, a silicon oxynitride film formed by a plasma CVD method may be used.

Next, treatment for adding an oxygen 431 (oxygen doping treatment) is performed on the oxide insulating film 480 having the projecting portion on its surface, so that an oxygen-excess region 481 is formed in the vicinity of the bottom surface of the oxide insulating film 480 and in the vicinity of the conductive layer 491. Thus, an oxide insulating film 484 having the oxygen-excess region 481 is formed (see FIG. 1B). Note that in the drawing, the oxygen-excess region 481 is illustrated by a dotted line, and the dotted line schematically indicates a center of distribution of the added oxygen.

At least any of an oxygen radical, an oxygen atom, an oxygen molecule, ozone, an oxygen ion (an oxygen molecular ion) and/or an oxygen cluster ion may be included in the oxygen 431.

Introducing the oxygen 431 into the oxide insulating film 480 can be performed by, for example, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like. Note that as an ion implantation method, a gas cluster ion beam may be used. The oxygen 431 may be introduced into the entire surface of the substrate 400 at a time. Alternatively, a linear ion beam may be used, for example. In the case of using a linear ion beam, relative movement (scanning) of the substrate or the ion beam enables the oxygen 431 to be introduced into the entire surface of the oxide insulating film 480.

As a supply gas of the oxygen 431, a gas containing oxygen (O) can be used; for example, an O₂ gas, an N₂O gas, a CO₂ gas, a CO gas, or an NO₂ gas can be used. Note that a rare gas (e.g., an Ar gas) may be contained in the supply gas of the oxygen.

In the case where oxygen is added by an ion implantation method, the dose of the oxygen 431 is preferably 0.5×10¹⁶ cm⁻² or higher and 5×10¹⁶ cm⁻² or lower (e.g., 1×10¹⁶ ions/cm²), and the acceleration energy is preferably 50 eV or higher and 70 eV or lower (e.g., 50 eV). The oxygen content in an oxide insulating film 436 including the oxygen-excess region 481 on which the oxygen doping treatment has been performed is preferably higher than that in the stoichiometric composition of the oxide insulating film 436. Such a region containing oxygen in excess of the stoichiometric composition preferably exist in the oxygen-excess region 481. The depth at which the oxygen 431 is implanted may be adjusted as appropriate by implantation conditions.

Next, planarization treatment is performed on the oxide insulating film 484 including the oxygen-excess region 481 so that the projecting portion of the top surface of the oxide insulating film 484 is removed. The oxide insulating film 480 over the conductive layer 491 is selectively removed, so that the surface of the oxide insulating film 480 is planarized. Accordingly, the planarized oxide insulating film 436 is formed (see FIG. 1C).

By the planarization treatment, the oxide insulating film 480 over the conductive layer 491 is selectively removed to be thin, whereby the distance between the oxygen-excess region 481 over the conductive layer 491 and the top surface of the oxide insulating film 480 becomes shorter. In contrast, a region of the oxide insulating film 480 in which the conductive layer 491 does not exist is hardly removed, and the oxygen-excess region 481 exists in the vicinity of the bottom surface of the oxide insulating film 480. Thus, in the oxide insulating film 436, the oxygen-excess region 481 is provided at a shallow position from the top surface of the oxide insulating film in the region where the conductive layer 491 exists and is provided at a deep position from the top surface of the oxide insulating film in another region (the region where the conductive layer 491 does not exist).

Since the oxygen-excess region 481 can be provided close to the oxide semiconductor film in a part of the oxide insulating film 436 over which the oxide semiconductor film is provided in a later step and which overlaps with the conductive layer 491, oxygen can be efficiently supplied from the oxygen-excess region 481 to the oxide semiconductor film. Supply of oxygen can be promoted by heat treatment.

In the oxide insulating film 436, the oxygen-excess region 481 is provided apart from the top surface of the oxide insulating film 436 in a region other than the region below the oxide semiconductor film to which oxygen is required to be supplied, that is, in the vicinity of the bottom surface of the oxide insulating film 436. Thus, in the case where heat treatment is performed, in particular, unnecessary release of oxygen from the top surface of the oxide insulating film 436 can be suppressed, and the oxide insulating film 436 can be kept in an oxygen excess state.

The addition of the oxygen 431 to the oxide insulating film 480 is described as an example in this embodiment; however, the addition of the oxygen 431 to the oxide insulating film 480 can be omitted as long as the oxide insulating film 480 contains sufficient oxygen shortly after the formation.

The planarization treatment may be, but not particularly limited to, polishing treatment (such as chemical mechanical polishing (CMP)), dry etching treatment, or plasma treatment.

Next, an oxide semiconductor film 403 is formed over the oxide insulating film 436. As a material of the oxide semiconductor film 403, for example, the following can be used: a two-component metal oxide such as an In—Zn-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide, a three-component metal oxide such as an In—Ga—Zn-based oxide (also referred to as IGZO), an In—Sn—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide, or a four-component metal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, or an In—Sn—Hf—Zn-based oxide.

Note that here, for example, an In—Ga—Zn-based oxide means an oxide containing In, Ga, and Zn, and there is no limitation on the composition ratio of In, Ga, and Zn. Further, the In—Ga—Zn-based oxide semiconductor may contain a metal element other than In, Ga, and Zn.

In this embodiment, as a target for forming the oxide semiconductor film 403 by a sputtering method, an oxide target containing In, Ga, and Zn in an atomic ratio of In:Ga:Zn=3:1:2 is used to form an In—Ga—Zn-based oxide film (IGZO film) having a thickness of 5 nm or greater and 30 nm or smaller.

The relative density (filling factor) of the metal oxide target is greater than or equal to 90% and less than or equal to 100%, preferably greater than or equal to 95% and less than or equal to 99.9%. With use of the metal oxide target with high filling factor, a dense oxide semiconductor film can be formed.

It is preferable that a high-purity gas from which impurities such as hydrogen, water, a hydroxyl group, and hydride are removed be used as a sputtering gas for the formation of the oxide semiconductor film 403.

The substrate is held in a deposition chamber kept under reduced pressure. Then, a sputtering gas in which hydrogen and moisture are removed is introduced into the deposition chamber from which remaining moisture is being removed, so that the oxide semiconductor film 403 is formed over the oxide insulating film 436 with the use of the target. To remove moisture remaining in the deposition chamber, an entrapment vacuum pump such as a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the deposition chamber which is evacuated with the cryopump, for example, a hydrogen atom, a compound containing a hydrogen atom, such as water (H₂O), (further preferably, also a compound containing a carbon atom), and the like are removed, whereby the concentration of impurities in the oxide semiconductor film 403 formed in the deposition chamber can be reduced.

The oxide semiconductor film 403 can be formed by processing a film-shaped oxide semiconductor film into an island shape by a photolithography process.

A resist mask for forming the island-shaped oxide semiconductor film 403 may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced.

It is preferable that the oxide semiconductor film 403 be the one which is highly purified and hardly contain impurities such as copper. In the process for manufacturing a transistor 440 a, steps in which these impurities are not mixed or attached to the surface of the oxide semiconductor film 403 are preferably selected as appropriate. In the case where the impurities are attached to the surface of the oxide semiconductor film 403, the impurities on the surface of the oxide semiconductor film 403 are preferably removed by exposure to oxalic acid or dilute hydrofluoric acid or plasma treatment (such as N₂O plasma treatment). Specifically, the concentration of copper in the oxide semiconductor film 403 is lower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to 1×10¹⁷ atoms/cm³.

Heat treatment may be performed in order to promote supply of oxygen from the oxide insulating film 436 to the oxide semiconductor film 403.

Next, a source electrode layer 405 a and a drain electrode layer 405 b which are electrically connected to the oxide semiconductor film 403 are formed. A variety of circuits can be formed by connection with another transistor or element with the use of the source electrode layer 405 a and the drain electrode layer 405 b.

The source electrode layer 405 a and the drain electrode layer 405 b can be formed in such a manner that a conductive film is formed by a sputtering method, an evaporation method, or the like and then is processed by an etching method.

As the conductive film used for the source electrode layer 405 a and the drain electrode layer 405 b, for example, a metal film containing an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W, or a metal nitride film containing any of the above elements as a component (e.g., a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film) can be used.

In this embodiment, a single-layer tungsten film is used, a resist mask is formed over the tungsten film, first etching is performed to remove the tungsten film to approximately half the thickness so that the tungsten film partly has a small thickness, and ashing is performed on the resist mask for reducing the area of the resist mask. After that, second etching is performed with the resist mask, which is reduced in size, so that the source electrode layer 405 a and the drain electrode layer 405 b each including projecting portions when seen in cross-section as illustrated in FIG. 1D are formed.

Subsequently, the resist mask, which is reduced in size, is removed. A cross-sectional view at this stage corresponds to FIG. 1D.

Next, a gate insulating film 402 is formed to cover the oxide semiconductor film 403, the source electrode layer 405 a, and the drain electrode layer 405 b.

The gate insulating film 402 has a thickness greater than or equal to 1 nm and less than or equal to 20 nm, for example, and can be formed by a sputtering method, an MBE method, a CVD method, a pulsed laser deposition method, an ALD method, or the like as appropriate. The gate insulating film 402 may be formed with a sputtering apparatus which performs film deposition on surfaces of a plurality of substrates set substantially perpendicular to a surface of a sputtering target.

A material of the gate insulating film 402 can be a silicon oxide film, a gallium oxide film, a gallium oxide zinc film, a Ga₂O₃ (Gd₂O₃) film, a zinc oxide film, an aluminum oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxynitride film, or a silicon nitride oxide film. As another material of the gate insulating film 402, an In—Ga—Zn-based oxide film having an insulating property can be given. The In—Ga—Zn-based oxide film having an insulating property may be formed under the following conditions: an oxide target containing In, Ga, and Zn in an atomic ratio of In:Ga:Zn=1:3:2 is used, the substrate temperature is room temperature, and a sputtering gas is an argon gas or a mixed gas of argon and oxygen.

In this embodiment, a gallium oxide film formed by a sputtering method is used as the gate insulating film 402. By using the gallium oxide film as the gate insulating film 402, the gate insulating film 402 contains the same constituent material as that of the oxide semiconductor film 403, and thus the interface state between the oxide semiconductor film 403 and the gate insulating film 402 can be excellent, which gives stable electrical characteristics.

Next, a conductive film is formed over the gate insulating film 402 by a sputtering method, an evaporation method, or the like. The conductive film is etched so that gate electrode layers 401 a and 401 b are formed.

The gate electrode layers 401 a and 401 b can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, or scandium or an alloy material which contains any of these materials as its main component. Alternatively, a semiconductor film typified by a polycrystalline silicon film doped with an impurity element such as phosphorus, or a silicide film such as a nickel silicide film may be used as the gate electrode layers 401 a and 401 b. The gate electrode layers 401 a and 401 b may have a single-layer structure or a stacked-layer structure.

In this embodiment, a metal oxide film containing nitrogen (an In—Ga—Zn-based oxide film containing nitrogen) is used as the gate electrode layer 401 a over and in contact with the gate insulating film 402, and a tungsten film is formed over the gate electrode layer 401 a as the gate electrode layer 401 b. The use of the metal oxide film containing nitrogen as the gate electrode layer 401 a enables the transistor to have a positive threshold voltage.

Through the above steps, the transistor 440 a of this embodiment can be manufactured (see FIG. 2A). The transistor 440 a is an example of a top-gate transistor. FIG. 2A is a cross-sectional view of the transistor 440 a in the channel length direction.

The conductive layer 491 provided in the oxide insulating film 436 including the oxygen-excess region 481 overlaps with the gate electrode layers 401 a and 401 b with a channel formation region positioned therebetween, and controls the electrical characteristics of the transistor 440 a.

The conductive layer 491 serves as a second gate electrode layer (what we call a back gate) which controls the electrical characteristics of the transistor 440 a. For example, by setting the potential of the conductive layer 491 to GND (or a fixed potential), the threshold voltage of the transistor 440 a is increased, so that the transistor 440 a can be normally off The gate electrode layers 401 a and 401 b overlap with part of the source electrode layer 405 a and part of the drain electrode layer 405 b. Since some of the projecting portions of the source electrode layer 405 a and the drain electrode layer 405 b have shapes with which coverage with the gate insulating film 402 can be improved, electric-field concentration is reduced, which contributes to improvement in the electrical characteristics and reliability of the transistor.

Next, an insulating film 407 covering the gate electrode layers 401 a and 401 b is formed (see FIG. 2B). The insulating film 407 functions as a barrier film (protective film) having a high property of preventing oxygen from being released from the oxide semiconductor film 403 or the gate insulating film 402.

The insulating film 407 functioning as a barrier film is preferably a fine film which can prevent impurities such as hydrogen and moisture from entering to the oxide semiconductor film 403.

As the insulating film 407 functioning as a barrier film, for example, an inorganic insulating film such as a gallium oxide film, a gallium oxide zinc film, a Ga₂O₃ (Gd₂O₃) film, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, an aluminum nitride oxide film, or a zinc oxide film can be used. The insulating film 407 may have a single layer or a stacked layer. The insulating film 407 functioning as a barrier film can be formed by a plasma CVD method, a sputtering method, a CVD method using a deposition gas, or an MBE method.

Next, an interlayer insulating film 485 is formed over the insulating film 407. The interlayer insulating film 485 may be a single layer or a stacked layer, and an inorganic insulating film such as a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, an aluminum nitride oxide film, a gallium oxide film, or a gallium oxide zinc film can be used.

FIG. 2C is a cross-sectional view illustrating the steps up to and including the formation of an embedded wiring in the interlayer insulating film 485. A contact hole reaching the source electrode layer 405 a is formed in the interlayer insulating film 485, a first barrier metal film 486 is formed, and a copper film or a copper alloy film is formed thereover so as to form a low-resistance conductive layer 487. Then, polishing is performed for planarization, and a second barrier metal film 488 is formed so as to protect the exposed low-resistance conductive layer 487. The embedded wiring includes the first barrier metal film 486, the second barrier metal film 488, and the low-resistance conductive layer 487 surrounded by the first barrier metal film 486 and the second barrier metal film 488.

Each of the first barrier metal film 486 and the second barrier metal film 488 may be formed using a conductive material which suppresses diffusion of copper contained in the low-resistance conductive layer 487. Examples of the conductive material are a tantalum nitride film, a molybdenum nitride film, and a tungsten nitride film.

Through the above steps, a semiconductor device having a multilayer structure in which another semiconductor element, another wiring, or the like is formed over the transistor 440 a can be manufactured. The semiconductor element, the wiring, or the like which is provided over the transistor 440 a can be electrically connected to the embedded wiring.

The cross-sectional structure of the transistor 440 a described in this embodiment is merely an example, and is not limited as long as the cross-sectional shapes of the source electrode layer 405 a and the drain electrode layer 405 b are the same as those in this embodiment. Although the transistor 440 a is a MOSFET here, an insulated gate bipolar transistor (IGBT), a metal semiconductor field effect transistor (MESFET), or the like can be used as the transistor 440 a. In the case where an IGBT is used as the transistor 440 a, a source electrode layer corresponds to an emitter terminal and a drain electrode layer corresponds to a collector terminal. Examples of cross-sectional structures of other transistors are given below.

A transistor 440 b illustrated in FIG. 3A has a structure in which the thickness of a region of the oxide semiconductor film 403 which does not overlap with the source electrode layer 405 a and the drain electrode layer 405 b and overlaps with the conductive layer 491 is smaller than the thickness of regions of the oxide semiconductor film 403 which overlap with the source electrode layer 405 a and the drain electrode layer 405 b. The structure in FIG. 3A is the same as that in FIG. 2B except a depressed portion is formed in the channel formation region. Thus, detailed description thereof is omitted here.

The oxide semiconductor film 403 of the transistor 440 b in FIG. 3A can be formed in such a manner that, after the source electrode layer 405 a and the drain electrode layer 405 b are formed, etching is performed with the use of the source electrode layer 405 a and the drain electrode layer 405 b as masks. The depressed portion formed in the channel formation region as illustrated in FIG. 3A makes it possible to electric-field concentration which might occur between the source electrode layer 405 a and the drain electrode layer 405 b to be reduced.

A transistor 440 c illustrated in FIG. 3B has a stacked-layer structure in which a second oxide semiconductor film 403 b having a composition different from that of a first oxide semiconductor film 403 a is formed over the first oxide semiconductor film 403 a. The second oxide semiconductor film 403 b includes a depressed portion. The structure in FIG. 3B is the same as that in FIG. 3A except the transistor 440 c has a stacked-layer structure. Thus, detailed description thereof is omitted here.

As the first oxide semiconductor film 403 a, an In—Ga—Zn-based oxide film formed with a target having an atomic ratio of In:Ga:Zn=3:1:2 is used. The first oxide semiconductor film 403 a preferably contains more In than Ga and Zn in an atomic ratio. As the second oxide semiconductor film 403 b, an In—Ga—Zn-based oxide film formed with a target having an atomic ratio of In:Ga:Zn=1:1:1 is used.

In the case where stacked oxide semiconductor films having different compositions are used, the stacked oxide semiconductor films are formed so that an upper film of the stacked films remains and thus a lower film of the stacked films (the first oxide semiconductor film 403 a in this embodiment) is not exposed to a bottom of the depressed portion.

An In—Ga—Zn-based oxide film formed with a target having an atomic ratio of In:Ga:Zn=3:1:2 is used for a channel formation region of the transistor 440 c in FIG. 3B, and thus field-effect mobility can be improved.

A transistor 440 d in FIG. 3C is an example in which an insulating film 437 containing gallium oxide is provided between an oxide insulating film 493 and the stacked oxide semiconductor films, and a conductive layer 492 does not overlap with the channel formation region. Oxygen is supplied to the stacked oxide semiconductor films from the insulating film 437 containing gallium oxide, which includes an oxygen-excess region containing oxygen whose proportion is higher than that in the stoichiometric composition. Thus, oxygen doping treatment is not performed on the oxide insulating film 493. Except for these different points, the structure in FIG. 3C is the same as that in FIG. 3B. Thus, detailed description thereof is omitted here.

In the case where the threshold voltage of the transistor 440 d is increased and the transistor 440 d is a normally-off transistor shortly after the formation thereof, as illustrated in FIG. 3C, the conductive layer 492 does not necessarily overlap with the channel formation region. In the case where the transistor 440 d is not required to be a normally-off transistor depending on the circuit configuration, only the transistor can be used as a normally-on transistor.

The insulating film 437 containing gallium oxide can be etched with the use of the photomask which is also used for the stacked oxide semiconductor films. The pattern shape of the insulating film 437 when seen from the top surface is the same as that of the stacked oxide semiconductor films. Thus, the structure in FIG. 3C can be obtained using the same number of masks as the structure in FIG. 3B.

In the transistor 440 d, in the case where an insulating film containing gallium oxide is used as the gate insulating film 402, the stacked oxide semiconductor films can be sandwiched by insulating films containing gallium oxide which are provided below and over and in contact with the oxide semiconductor films. The insulating film containing gallium oxide prevents oxygen from being released from the stacked oxide semiconductor films and effectively supplies oxygen to the stacked oxide semiconductor films. The insulating films provided below and over the oxide semiconductor films contain the same constituent material as that of the oxide semiconductor films, and thus the interface state between the oxide semiconductor films and the insulating films can be excellent, which gives stable electrical characteristics.

The structures of the transistors illustrated in FIG. 2B and FIGS. 3A to 3C are partly different from one another; however, the present invention is not limited to these structures, and various combinations are possible. For example, by combining the planarized oxide semiconductor film 403 illustrated in FIG. 2B and the insulating film 437 containing gallium oxide illustrated in FIG. 3C, a transistor of one embodiment of the present invention may have a structure in which the planarized oxide semiconductor film 403 is provided over the insulating film 437 containing gallium oxide.

Embodiment 2

In this embodiment, an example of a method for manufacturing a transistor is described below; the method allows reduction in parasitic capacitance that is formed among a gate electrode layer, a gate insulating film, and a source electrode layer which overlaps with the gate electrode layer with the gate insulating film provided therebetween and parasitic capacitance that is formed among the gate electrode layer, the gate insulating film, and a drain electrode layer which overlaps with the gate electrode layer with the gate insulating film provided therebetween. Part of steps in a manufacturing process is the same as that in Embodiment 1, and thus the detailed description of the part of the steps is omitted.

First, steps up to and including the step in FIG. 1C, which is described in Embodiment 1, are performed. That is, the conductive layer 491 is formed over the substrate 400, and the oxide insulating film 436 including the oxygen-excess region 481 is formed. A cross-sectional view at this stage is shown in FIG. 4A. Note that FIG. 4A is the same as FIG. 1C.

Then, the first oxide semiconductor film 403 a is formed over the oxide insulating film 436, and the second oxide semiconductor film 403 b is formed over the first oxide semiconductor film 403 a. The first oxide semiconductor film 403 a and the second oxide semiconductor film 403 b are successively formed without exposure to the air.

As the first oxide semiconductor film 403 a, an In—Ga—Zn-based oxide film formed with a target having an atomic ratio of In:Ga:Zn=3:1:2 is used. The first oxide semiconductor film 403 a contains more In than Ga and Zn in an atomic ratio. As the second oxide semiconductor film 403 b, an In—Ga—Zn-based oxide film formed with a target having an atomic ratio of In:Ga:Zn=1:1:1 is used.

Next, the oxide semiconductor films are processed into island-shaped oxide semiconductor films by a photolithography process. After that, a conductive film is formed by a sputtering method, an evaporation method, or the like.

Next, resist masks 408 a and 408 b are formed over the conductive film, and etching is performed to remove the conductive film to approximately half the thickness so that the conductive film partly has a small thickness. Thus, a conductive film 406 which partly has a small thickness is formed. A cross-sectional view at this stage is illustrated in FIG. 4B.

Subsequently, the resist masks 408 a and 408 b are removed and then, a protective layer 409 is formed over the conductive film 406 which partly has a small thickness (see FIG. 4C). This protective layer 409 is provided to protect part of the conductive film in later etching of the gate insulating film 402. This protective layer 409 is formed using a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, an aluminum nitride oxide film, or the like. Note that is preferable that the protective layer 409 be formed using a material different from that of the gate insulating film 402 and etching selectivity of the gate insulating film 402 to the protective layer 409 be high. In this embodiment, a silicon oxide film formed by a sputtering method is used.

Next, a resist mask is formed over the protective layer 409, and then the source electrode layer 405 a and the drain electrode layer 405 b each including projecting portions when seen in cross-section as illustrated in FIG. 4D are formed. In the formation of the source electrode layer 405 a and the drain electrode layer 405 b, the protective layer 409 is also etched, and a first protective layer 410 a which overlaps with and is in contact with the source electrode layer 405 a and a second protective layer 410 b which overlaps with and is in contact with the drain electrode layer 405 b are formed. A cross-sectional view at the stage in which the resist mask is removed is illustrated in FIG. 4D.

Then, the gate insulating film 402 covering the first oxide semiconductor film 403 a, the second oxide semiconductor film 403 b, the first protective layer 410 a, and the second protective layer 410 b is formed. As a material of the gate insulating film 402, a film containing gallium oxide, typically a gallium oxide film, is used in this embodiment. Even in the case where the film containing gallium oxide has a small thickness such as 20 nm or smaller, the film prevents impurities such as nitrogen from entering the first oxide semiconductor film 403 a or the second oxide semiconductor film 403 b provided below the film when or after a metal oxide film containing nitrogen is formed by a sputtering method or the like in a later step.

Next, a conductive film is formed over the gate insulating film 402 by a sputtering method, an evaporation method, or the like. The conductive film is etched so that the gate electrode layers 401 a and 401 b are formed. Next, part of the gate insulating film 402 is removed with the use of the gate electrode layers 401 a and 401 b as masks. When part of the gate insulating film 402 is removed, the first protective layer 410 a and the second protective layer 410 b function as etching stoppers, that is, protect the source electrode layer 405 a and the drain electrode layer 405 b from etching treatment.

In this embodiment, a metal oxide film containing nitrogen (an In—Ga—Zn-based oxide film containing nitrogen) is used as the gate electrode layer 401 a over and in contact with the gate insulating film 402, and a tungsten film is formed over the metal oxide as the gate electrode layer 401 b.

Through the above steps, a transistor 441 a of this embodiment can be manufactured. Lastly, the insulating film 407 is formed so that the insulating film 407 covers the gate electrode layers 401 a and 401 b and is in contact with the first protective layer 410 a and the second protective layer 410 b (see FIG. 4E). The insulating film 407 functions as a barrier film having a high property of preventing oxygen from being released from the first oxide semiconductor film 403 a, the second oxide semiconductor film 403 b, or the gate insulating film 402. The transistor 441 a is an example of a top-gate transistor. FIG. 4E is a cross-sectional view of the transistor 441 a in the channel length direction.

In the transistor 441 a in FIG. 4E, even though the gate electrode layers 401 a and 401 b overlap with part of the source electrode layer 405 a and part of the drain electrode layer 405 b, since the first protective layer 410 a and the second protective layer 410 b are provided over and in contact with the source electrode layer 405 a and the drain electrode layer 405 b, respectively, parasitic capacitance of this portion is smaller than that of the transistor 440 a in Embodiment 1.

The cross-sectional structure of the transistor 441 a described in this embodiment is merely an example. The cross-sectional structure is not limited as long as the cross-sectional shapes of the source electrode layer 405 a and the drain electrode layer 405 b are the same as those in this embodiment. An example of a cross-sectional structure of another transistor is given below.

A transistor 441 b in FIG. 5A is an example in which an insulating film 438 containing gallium oxide is provided between the oxide insulating film 436 and the first oxide semiconductor film 403 a.

The transistor 441 b in FIG. 5A includes the insulating film 438 containing gallium oxide in contact with bottom surfaces of the source electrode layer 405 a and the drain electrode layer 405 b. The insulating film 438 containing gallium oxide functions as an etching stopper for protecting the oxide insulating film 436 in etching of the gate insulating film 402. The insulating film 407 is in contact with the insulating film 438 containing gallium oxide in the periphery region of the transistor.

The structure in FIG. 5A is the same as that in FIG. 4E except the insulating film 438 containing gallium oxide is provided between the oxide insulating film 436 and the first oxide semiconductor film 403 a. Thus, detailed description thereof is omitted here.

A transistor 441 c in FIG. 5B includes the insulating film 438 containing gallium oxide which is provided between the oxide insulating film 436 and the first oxide semiconductor film 403 a and is partly in contact with the gate insulating film 402. The transistor in FIG. 5B has the same structure as that in FIG. 5A except for the top shape of the gate insulating film 402. Thus, detailed description thereof is omitted here.

In the transistor 441 c, since the gate insulating film 402 contains gallium oxide, the first oxide semiconductor film 403 a, the second oxide semiconductor film 403 b, the source electrode layer 405 a, and the drain electrode layer 405 b are covered with insulating films containing gallium oxide. Thus, oxygen is prevented from being released all around from the stacked oxide semiconductor films.

A transistor 441 d illustrated in FIG. 5C includes the insulating film 437 containing gallium oxide between an insulating film 434 and the first oxide semiconductor film 403 a, and has a stacked-layer structure in which the second oxide semiconductor film 403 b having a composition different from that of the first oxide semiconductor film 403 a is formed over the first oxide semiconductor film 403 a. The second oxide semiconductor film 403 b includes a depressed portion. Oxygen is supplied to the stacked oxide semiconductor films from the insulating film 437 containing gallium oxide, which includes an oxygen-excess region where the proportion of oxygen is higher than that in the oxide semiconductor film in the stoichiometric composition exists. Thus, oxygen doping treatment is not performed on the oxide insulating film 435. The insulating film 434 covering the conductive layer 491 is formed, an oxide insulating film 435 is polished to expose part of the insulating film 434, and the insulating film 437 containing gallium oxide is provided so as to overlap with the exposed portion.

The insulating film 434 is a barrier film, and is formed using an inorganic insulating film such as an aluminum oxide film, an aluminum oxynitride film, a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, an aluminum nitride oxide film, a gallium oxide film, or a gallium zinc oxide film.

The insulating film 434 is a silicon oxide film or a silicon oxynitride film obtained by a plasma CVD method.

The transistor 441 d in FIG. 5C has the same structure as that in FIG. 5A except the insulating film 437 containing gallium oxide, the second oxide semiconductor film 403 b including a depressed portion, and the insulating film 434 are provided and oxygen doping treatment is not performed. Thus, detailed description thereof is omitted here.

A transistor 441 e in FIG. 6A has the same structure as that in FIG. 5C except the second oxide semiconductor film 403 b has a flat top surface. Thus, detailed description thereof is omitted here.

FIG. 6B illustrates an example of a top view of the transistor 441 e. A cross-sectional view taken along a chain line A-B in FIG. 6B corresponds to FIG. 6A. As illustrated in FIG. 6B, the source electrode layer 405 a or the drain electrode layer 405 b covers the periphery of the second oxide semiconductor film 403 b, and the gate electrode layer 401 b covers a region of the second oxide semiconductor film 403 b which is not covered with the source electrode layer 405 a or the drain electrode layer 405 b. Thus, the second oxide semiconductor film 403 b is not exposed in etching of the gate electrode layer 401 b. As illustrated in FIG. 6A, the top surface of the source electrode layer 405 a is covered with the first protective layer 410 a and the top surface of the drain electrode layer 405 b is covered with the second protective layer 410 b; thus, the source electrode layer 405 a and the drain electrode layer 405 b are not removed in etching of the gate electrode layer 401 b.

The structures of the transistors illustrated in FIG. 4E, FIGS. 5A to 5C, and FIG. 6A are partly different from one another; however, the present invention is not limited to these structures, and various combinations are possible.

Further, this embodiment can be freely combined with Embodiment 1.

It is needless to say that in this embodiment, the same reference numerals and the same materials are used for the same parts as those in Embodiment 1.

Embodiment 3

-   -   In this embodiment, an example of a semiconductor device         including the transistors described in Embodiment 1 or 2 is         described with reference to FIGS. 7A and 7B.

The semiconductor device illustrated in FIGS. 7A and 7B includes transistors 740 and 750 including a first semiconductor material in a lower portion, and a transistor 610 including a second semiconductor material in an upper portion. The transistor 610 has a structure similar to that of the transistor 441 e described in Embodiment 2. The same reference numerals are used for the same parts as those in FIGS. 6A and 6B. FIG. 7B is a circuit diagram of the semiconductor device in FIG. 7A.

Here, the first semiconductor material and the second semiconductor material are preferably materials having different band gaps. For example, the first semiconductor material may be a semiconductor material other than an oxide semiconductor (e.g., silicon) and the second semiconductor material may be an oxide semiconductor. A transistor including a material such as silicon can easily operate at high speed. On the other hand, charge can be held in a transistor including an oxide semiconductor for a long time owing to its characteristics.

As a substrate used in the semiconductor device, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon or silicon carbide, a compound semiconductor substrate made of silicon germanium or the like, a silicon on insulator (SOI) substrate, or the like can be used. A channel formation region of the transistor can be formed in or over the semiconductor substrate. The semiconductor device in FIG. 7A is an example in which the channel formation region is formed in the semiconductor substrate to form a lower transistor.

In the semiconductor device in FIG. 7A, a single crystal silicon substrate is used as a substrate 700, and the transistors 740 and 750 are formed using the single crystal silicon substrate. Single crystal silicon is used as the first semiconductor material. The transistor 740 is an n-channel transistor and the transistor 750 is a p-channel transistor. The transistor 740 and the transistor 750 which are electrically connected to each other form a complementary metal oxide semiconductor (CMOS) circuit 760.

In this embodiment, the single crystal silicon substrate imparting p-type conductivity is used as the substrate 700; thus, an n-well is formed by adding an impurity element imparting n-type conductivity to a region in which the p-channel transistor 750 is to be formed. A channel formation region 753 of the transistor 750 is formed in the n-well. As the impurity element imparting n-type conductivity, phosphorus (P), arsenic (As), or the like can be used.

Here, an impurity element imparting p-type conductivity is not added to a region in which the n-channel transistor 740 is to be formed; however, a p-well may be formed by adding an impurity element imparting p-type conductivity. As the impurity element imparting p-type conductivity, boron (B), aluminum (Al), gallium (Ga), or the like may be used.

Meanwhile, when a single-crystal silicon substrate imparting n-type conductivity is used, an impurity element imparting p-type conductivity may be added to form a p-well.

The transistor 740 includes a channel formation region 743, an n-type impurity region 744 functioning as a lightly doped drain (LDD) region or an extension region, an n-type impurity region 745 functioning as a source region or a drain region, a gate insulating film 742, and a gate electrode layer 741. The n-type impurity region 745 has a higher impurity concentration than the n-type impurity region 744. A side surface of the gate electrode layer 741 is provided with a sidewall insulating layer 746. With the use of the gate electrode layer 741 and the sidewall insulating layer 746 as masks, the n-type impurity region 744 and the n-type impurity region 745 which have different impurity concentrations can be formed in a self-aligned manner.

The transistor 750 includes the channel formation region 753, a p-type impurity region 754 functioning as a lightly doped drain (LDD) region or an extension region, a p-type impurity region 755 functioning as a source region or a drain region, a gate insulating film 752, and a gate electrode layer 751. The p-type impurity region 755 has a higher impurity concentration than the p-type impurity region 754. A side surface of the gate electrode layer 751 is provided with a sidewall insulating layer 756. With the use of the gate electrode layer 751 and the sidewall insulating layer 756 as masks, the p-type impurity region 754 and the p-type impurity region 755 which have different impurity concentrations can be formed in a self-aligned manner.

In the substrate 700, the transistor 740 and the transistor 750 are isolated from each other by an element isolation region 789. An insulating film 788 and an insulating film 687 are stacked over the transistor 740 and the transistor 750. A wiring layer 647 electrically connected to the n-type impurity region 745 through an opening in the insulating film 788 and the insulating film 687 and a wiring layer 657 electrically connected to the p-type impurity region 755 through an opening in the insulating film 788 and the insulating film 687 are provided over the insulating film 687. A wiring layer 748 is provided over the insulating film 687 so as to electrically connect the transistor 740 and the transistor 750. The wiring layer 748 is electrically connected to the n-type impurity region 745 through an opening in the insulating film 788 and the insulating film 687 and reaching the n-type impurity region 745. Further, the wiring layer 748 is electrically connected to the p-type impurity region 755 through an opening in the insulating film 788 and the insulating film 687 and reaching the p-type impurity region 755.

An insulating film 686 is provided over the insulating film 687, the wiring layer 647, the wiring layer 748, and the wiring layer 657. A wiring layer 658 is formed over the insulating film 686. The wiring layer 658 is electrically connected to a gate wiring through an opening in the insulating films 788, 687, and 686. The gate wiring is formed over the gate insulating film 742 and the channel formation region 753. The gate wiring branches into the gate electrode layer 741 and the gate electrode layer 751.

The semiconductor device of this embodiment is not limited to the structure in FIG. 7A. As the transistors 740 and 750, a transistor containing silicide or a transistor which does not include a sidewall insulating layer may be used. When a structure that contains silicide is used, resistance of the source region and the drain region can be further lowered and the speed of the semiconductor device is increased. Further, the semiconductor device can be operated at low voltage; thus, power consumption of the semiconductor device can be reduced.

Next, the structures of upper elements provided over the lower transistor in the semiconductor device in FIGS. 7A and 7B are described.

An insulating film 684 is stacked over the insulating film 686 and the wiring layer 658. The conductive layer 491 and a wiring layer 692 are formed over the insulating film 684. The insulating film 434 is provided so as to cover the conductive layer 491 and the wiring layer 692. The oxide insulating film 435 is provided over the insulating film 434. The insulating film 437 containing gallium oxide, the first oxide semiconductor film 403 a, and the second oxide semiconductor film 403 b having a composition different from that of the first oxide semiconductor film 403 a are stacked in this order over the oxide insulating film 435. The source electrode layer 405 a and the drain electrode layer 405 b each including projecting portions are provided over the second oxide semiconductor film 403 b. The first protective layer 410 a which overlaps with and is in contact with the source electrode layer 405 a and the second protective layer 410 b which overlaps with and is in contact with the drain electrode layer 405 b are provided. The gate insulating film 402 is provided over and in contact with a region of the second oxide semiconductor film 403 b which overlaps with neither the source electrode layer 405 a nor the drain electrode layer 405 b (a channel formation region). The gate electrode layers 401 a and 401 b are provided over the gate insulating film 402.

The capacitor 690 is also formed over the insulating film 435 by the same process as that of the transistor 610. The source electrode layer 405 a serves as one electrode of the capacitor 690. Capacitor electrode layers 693 a and 693 b serve as the other electrode of the capacitor 690. The first protective layer 410 a and an insulating film 682, which is formed in the same step as the gate insulating film 402, which are provided between the source electrode layer 405 a and the capacitor electrode layer 693 a serve as dielectrics. The capacitor electrode layers 693 a and 693 b are formed in the same step as the gate electrode layers 401 a and 401 b.

By setting the potential of the conductive layer 491 to GND (or a fixed potential), the conductive layer 491 serves as a back gate which controls the electrical characteristics of the transistor 610. The conductive layer 491 has a function of preventing static electricity. In the case where the threshold voltage of the transistor 610 is not required to be controlled by the conductive layer 491 in order to make the transistor 610 be a normally-off transistor, the conductive layer 491 is not necessarily provided. In the case where the transistor 610 is used for part of a particular circuit and a problem might be caused by providing the conductive layer 491, the conductive layer 491 is not necessarily provided in the circuit.

The wiring layer 692 is electrically connected to the wiring layer 658 through an opening in the insulating film 684. In this embodiment, planarization treatment using a CMP method is performed on the insulating film 684.

The insulating film 434 is provided between the lower portion and the upper portion in the semiconductor device. The insulating film 434 functions as a barrier film for preventing impurities such as hydrogen which cause degradation or variation in electrical characteristics of the transistor 610 from entering the upper portion from the lower portion. Thus, the insulating film 434 is preferably formed using a fine organic insulating film (e.g., an aluminum oxide film or a silicon nitride film) having a high blocking property of impurities or the like.

In the case of using the same manufacturing method as that described in Embodiment 2, the transistor 610 can be manufactured similarly to the transistor 441 e. The method for manufacturing the transistor 610 is briefly described.

The conductive layer 491 and the wiring layer 692 are formed over the insulating film 684 which is provided over the transistors 740 and 750.

Next, the insulating film 434 is formed so as to cover the conductive layer 491 and the wiring layer 692.

Then, an oxide insulating film having projecting portions reflecting the shapes of the conductive layer 491 and the wiring layer 692 is formed over the insulating film 434. CMP treatment is performed on the oxide insulating film, whereby the oxide insulating film over the conductive layer 491 and the wiring layer 692 is selectively removed and the surface of the oxide insulating film is planarized. Accordingly, the planarized oxide insulating film 435 is formed.

Subsequently, part of the insulating film 434 over the top surface of the wiring layer 692 is selectively removed, so that an opening for exposing the top surface of the wiring layer 692 is formed.

Next, the insulating film 437 containing gallium oxide, the first oxide semiconductor film 403 a, and the second oxide semiconductor film 403 b are successively formed by a sputtering method without exposure to the air, and are selectively etched with one photomask.

A gallium oxide film is used as the insulating film 437 containing gallium oxide. The first oxide semiconductor film 403 a is formed to be what we call a CAAC-OS film using an oxide target having an atomic ratio of In:Ga:Zn=3:1:2 in an oxygen atmosphere (oxygen: 100%). The film includes a crystal part in which c-axes are aligned in the direction parallel to a normal vector of a surface where the film is formed or a normal vector of a surface of the film, triangular or hexagonal atomic arrangement which is seen from the direction perpendicular to the a-b plane is formed, and metal atoms are arranged in a layered manner or metal atoms and oxygen atoms are arranged in a layered manner when seen from the direction perpendicular to the c-axis. The second oxide semiconductor film 403 b is formed to be a CAAC-OS film using an oxide target having an atomic ratio of In:Ga:Zn=1:1:1 in an oxygen atmosphere (oxygen: 100%). The sum of the thicknesses of the first oxide semiconductor film 403 a and the second oxide semiconductor film 403 b of the completed transistor is greater than or equal to 5 nm and less than or equal to 10 nm. An example in which the oxide semiconductor film includes crystal parts shortly after the oxide semiconductor film is formed is described in this embodiment; however, crystal parts may be formed by performing heat treatment after the film formation.

A conductive film is formed over the second oxide semiconductor film 403 b by a sputtering method or the like, and a silicon oxide film is formed over the conductive film. Next, a resist mask is formed over the silicon oxide film, and first etching is performed to remove the conductive film to approximately half the thickness so that the silicon oxide film partly has a small thickness. Ashing is performed on the resist mask for reducing the area of the resist mask. After that, second etching is performed with the resist mask, which is reduced in size, so that the source electrode layer 405 a and the drain electrode layer 405 b each including projecting portions are formed. The first protective layer 410 a remains over a region of the source electrode layer 405 a which has a large thickness, and the second protective layer 410 b remains over a region of the drain electrode layer 405 b which has a large thickness. The source electrode layer 405 a is electrically connected to the wiring layer 692 through the opening in the insulating film 434.

Next, the gate insulating film 402 is formed over the second oxide semiconductor film 403 b, the first protective layer 410 a, and the second protective layer 410 b. As a material of the gate insulating film 402, a gallium oxide film is used in this embodiment.

Next, a conductive film is formed over the gate insulating film 402 by a sputtering method, an evaporation method, or the like. The conductive film is etched so that the gate electrode layers 401 a and 401 b and the capacitor electrode layers 693 a and 693 b are formed. Next, part of the gate insulating film 402 is removed with the use of the gate electrode layers 401 a and 401 b as masks. In the same step, part of the gate insulating film 402 is removed with the use of the capacitor electrode layers 693 a and 693 b as masks, so that the insulating film 682 is formed. When part of the gate insulating film 402 which is a gallium oxide film is removed, the first protective layer 410 a and the second protective layer 410 b which are silicon oxide films function as etching stoppers, that is, protect the source electrode layer 405 a and the drain electrode layer 405 b from etching treatment.

Through the above steps, the transistor 610 and the capacitor 690 are completed. The first oxide semiconductor film 403 a is formed over and in contact with the insulating film 437 which is a gallium oxide film, and the gate insulating film 402, which is a gallium oxide film, is formed over and in contact with the second oxide semiconductor film 403 b; thus, oxygen can be efficiently supplied to the first oxide semiconductor film 403 a and the second oxide semiconductor film 403 b. By providing the insulating film 437 and the gate insulating film 402, which are gallium oxide films, unnecessary release of oxygen can be suppressed, and the first oxide semiconductor film 403 a can be kept in an oxygen excess state. Thus, in the transistor 610, oxygen vacancies in the first oxide semiconductor film 403 a and at the interface between the insulating film 437 and the first oxide semiconductor film 403 a can be compensated efficiently. Even in the case where the gate insulating film 402, which is a gallium oxide film, has a small thickness such as 20 nm or smaller and the metal oxide film containing nitrogen is formed by a sputtering method in a later step, the gate insulating film 402 prevents impurities such as nitrogen from entering the second oxide semiconductor film 403 b provided below the film when or after the formation.

Next, the insulating film 407 and the interlayer insulating film 485 are formed over the transistor 610 and the capacitor 690. A cross-sectional view at this stage corresponds to FIG. 7A. Further, as described in Embodiment 1, a semiconductor device having a multilayer structure in which an embedded wiring is formed in the interlayer insulating film 485 and another semiconductor element, another wiring, or the like is formed above the embedded wiring may be manufactured.

Further, this embodiment can be freely combined with Embodiment 1 or 2.

Embodiment 4

As another example of a semiconductor device including the transistors described in Embodiment 1 or 2, a cross-sectional view of a NOR circuit, which is a logic circuit, is illustrated in FIG. 8A. FIG. 8B is a circuit diagram of the NOR circuit in FIG. 8A, and FIG. 8C is a circuit diagram of a NAND circuit.

In the NOR circuit illustrated in FIGS. 8A and 8B, p-channel transistors 801 and 802 each have a structure similar to that of the transistor 750 in FIGS. 7A and 7B in that a single crystal silicon substrate is used for a channel formation region, and n-channel transistors 803 and 804 each have a structure similar to that of the transistor 610 in FIGS. 7A and 7B and that of the transistor 441 e in Embodiment 1 in that an oxide semiconductor film is used for a channel formation region.

In the transistors 803 and 804 in the NOR circuit illustrated in FIGS. 8A and 8B, the conductive layer 491 is provided so as to overlap with a gate electrode layer with an oxide semiconductor film provided therebetween in order to control the electrical characteristics of the transistors. By controlling the potential of the conductive layer to GND, for example, the threshold voltages of the transistors 803 and 804 are increased, so that the transistors can be normally off. In the NOR circuit in this embodiment, conductive layers which are provided in the transistors 803 and 804 and can function as back gates are electrically connected to each other. However, the present invention is not limited to the structure, and the conductive layers functioning as back gates may be electrically controlled independently.

In the semiconductor device illustrated in FIG. 8A, a single crystal silicon substrate is used as a substrate 800, the transistor 802 is formed using the single crystal silicon substrate, and the transistor 803 in which stacked oxide semiconductor films are used for a channel formation region is formed over the transistor 802.

The gate electrode layers 401 a and 401 b of the transistor 803 are electrically connected to a wiring layer 832. The wiring layer 832 is electrically connected to a wiring layer 835. The gate electrode layers 401 a and 401 b of the transistor 803 are electrically connected to an embedded wiring, and the embedded wiring is electrically connected to a conductive layer 842. Note that the embedded wiring includes the first barrier metal film 486, the second barrier metal film 488, and the low-resistance conductive layer 487 covered by the first barrier metal film 486 and the second barrier metal film 488. The method for forming the embedded wiring is described in Embodiment 1. Thus, detailed description thereof is omitted here.

The wiring layer 832 is formed in an opening in insulating films 826 and 830, the wiring layer 835 is formed in an opening in an insulating film 833, and the conductive layer 842 is formed in an opening in the insulating film 434.

An electrode layer 825 of the transistor 802 is electrically connected to an electrode layer 845 b of the transistor 803 through wiring layers 831 and 834. The wiring layer 831 is formed in an opening in the insulating film 830, the wiring layer 834 is formed in an opening in the insulating film 833, and the electrode layer 845 b is formed in an opening in the insulating film 434. An electrode layer 845 a and the electrode layer 845 b function as source and drain electrode layers of the transistor 803.

The first oxide semiconductor film 403 a is formed over and in contact with the insulating film 437 which is a gallium oxide film, and the gate insulating film 402, which is a gallium oxide film, is formed over and in contact with the second oxide semiconductor film 403 b; thus, oxygen can be efficiently supplied to the first oxide semiconductor film 403 a and the second oxide semiconductor film 403 b. By providing the insulating film 437 and the gate insulating film 402, which are gallium oxide films, unnecessary release of oxygen can be suppressed, and the first oxide semiconductor film 403 a can be kept in an oxygen excess state. Thus, in the transistor 803, oxygen vacancies in the first oxide semiconductor film 403 a and at the interface between the insulating film 437 and the first oxide semiconductor film 403 a can be compensated efficiently. The transistor 804 has a structure and an effect which are similar to those of the transistor 803.

In the NAND circuit in FIG. 8C, p-channel transistors 811 and 814 each have a structure similar to that of the transistor 750 in FIGS. 7A and 7B, and n-channel transistors 812 and 813 each have a structure similar to that of the transistor 610 in FIGS. 7A and 7B in that an oxide semiconductor film is used for a channel formation region.

In the transistors 812 and 813 in the NAND circuit illustrated in FIG. 8C, a conductive layer is provided so as to overlap with a gate electrode layer with an oxide semiconductor film provided therebetween in order to control the electrical characteristics of the transistors. By controlling the potential of the conductive layer to GND, for example, the threshold voltages of the transistors 812 and 813 are increased, so that the transistors can be normally off. In the NAND circuit in this embodiment, conductive layers which are provided in the transistors 812 and 813 and function as back gates are electrically connected to each other. However, the present invention is not limited to the structure, and the conductive layers functioning as back gates may be electrically controlled independently.

By applying a transistor including an oxide semiconductor for a channel formation region and having extremely small off-state current to the semiconductor device in this embodiment, power consumption of the semiconductor device can be sufficiently reduced.

A semiconductor device which is miniaturized, is highly integrated, and has stable and excellent electrical characteristics by stacking semiconductor elements including different semiconductor materials and a method for manufacturing the semiconductor device can be provided.

The NOR circuit and the NAND circuit including the transistors described in Embodiment 1 or 2 are described as examples in this embodiment; however, the present invention is not limited to the circuits, and an AND circuit, an OR circuit, or the like can be formed using the transistors described in Embodiment 1 or 2. For example, a semiconductor device (storage device) in which stored data can be held even when power is not supplied and which has an unlimited number of times of writing with the transistors described in Embodiment 1 or 2 can be manufactured.

FIG. 9 is an example of a circuit diagram of a semiconductor device.

In FIG. 9, a first wiring (1st Line) is electrically connected to a source electrode layer of a transistor 160. A second wiring (2nd Line) is electrically connected to a drain electrode layer of the transistor 160. Any of the transistors 740, 750, and 802 described in this embodiment can be used as the transistor 160.

A third wiring (3rd Line) is electrically connected to one of a source and a drain electrode layers of a transistor 162, and a fourth wiring (4th Line) is electrically connected to a gate electrode layer of the transistor 162. A gate electrode layer of the transistor 160 and one of the source and drain electrode layers of the transistor 162 are electrically connected to one electrode of a capacitor 164. A fifth wiring (5th Line) is electrically connected to the other electrode of the capacitor 164.

For the transistor 162, any of the structures of the transistors 440 a, 440 b, 440 c, 440 d, 441 a, 441 b, 441 c, 441 d, and 441 e described in Embodiment 1 or 2 can be used.

The semiconductor device having the circuit configuration in FIG. 9 utilizes a characteristic in which the potential of the gate electrode layer of the transistor 160 can be held, and thus enables data writing, holding, and reading as follows.

Writing and holding of data are described. First, the potential of the fourth wiring is set to a potential at which the transistor 162 is turned on, so that the transistor 162 is turned on. Accordingly, the potential of the third wiring is supplied to the gate electrode layer of the transistor 160 and to the capacitor 164. That is, predetermined charge is supplied to the gate electrode layer of the transistor 160 (writing). Here, one of two kinds of charges providing different potentials (hereinafter referred to as a low-level charge and a high-level charge) is applied. After that, the potential of the fourth wiring is set to a potential at which the transistor 162 is turned off, so that the transistor 162 is turned off. Thus, the charge supplied to the gate electrode layer of the transistor 160 is held (holding).

Since the off-state current of the transistor 162 is extremely small, the charge of the gate electrode layer of the transistor 160 is held for a long time.

Next, reading of data is described. By supplying an appropriate potential (a reading potential) to the fifth wiring while supplying a predetermined potential (a constant potential) to the first wiring, the potential of the second wiring varies depending on the amount of charge held at the gate electrode layer of the transistor 160. This is because in general, when the transistor 160 is an n-channel transistor, an apparent threshold voltage V_(th) _(—) _(H) in the case where the high-level potential is given to the gate electrode layer of the transistor 160 is lower than an apparent threshold voltage V_(th) _(—) _(L) in the case where the low-level charge is given to the gate electrode layer of the transistor 160. Here, an apparent threshold voltage refers to the potential of the fifth wiring which is needed to turn on the transistor 160. Thus, the potential of the fifth wiring is set to a potential V₀ between V_(th) _(—) _(H) and V_(th) _(—) _(L), whereby charge supplied to the gate electrode layer of the transistor 160 can be determined For example, in the case where the high-level charge is supplied in writing, when the potential of the fifth wiring is V₀ (>V_(th) _(—) _(H)), the transistor 160 is turned on. In the case where the low-level charge is supplied in writing, even when the potential of the fifth wiring is V₀ (<V_(th) _(—) _(L)), the transistor 160 remains off. Therefore, the held data can be read by measuring the potential of the second wiring.

Note that in the case where memory cells are arrayed to be used, only data of desired memory cells needs to be read. In the case where such reading is not performed, a potential at which the transistor 160 is turned off regardless of the state of the gate electrode layer of the transistor 160, that is, a potential smaller than V_(th) _(—) _(H) may be given to the fifth wiring. Alternatively, a potential at which the transistor 160 is turned on regardless of the state of the gate electrode layer, that is, a potential higher than V_(th) _(—) _(L) may be given to the fifth line.

FIG. 10 illustrates another example of one embodiment of the structure of the storage device.

FIG. 10 is a perspective view of a memory device. The memory device illustrated in FIG. 10 includes a plurality of layers of memory cell arrays (memory cell arrays 3400(1) to 3400(n) (n is an integer greater than or equal to 2)) each including a plurality of memory cells as memory circuits in the upper portion, and a logic circuit 3004 in the lower portion which is necessary for operating the memory cell arrays 3400(1) to 3400(n).

FIG. 10 illustrates the logic circuit 3004, the memory cell array 3400(1), and the memory cell array 3400(2), and illustrates a memory cell 3170 a and a memory cell 3170 b as typical examples among the plurality of memory cells included in the memory cell array 3400(1) and the memory cell array 3400(2). The memory cell 3170 a and the memory cell 3170 b can have a configuration similar to the circuit configuration described in this embodiment with reference to FIG. 9, for example.

A transistor in which a channel formation region is formed in an oxide semiconductor film is used as each transistor included in the memory cells 3170 a and 3170 b. The structure of the transistor in which the channel formation region is formed in the oxide semiconductor film is the same as the structure described in Embodiment 1; thus, the description of the structure is omitted.

The logic circuit 3004 includes a transistor in which a semiconductor material other than an oxide semiconductor is used as a channel formation region. For example, the transistor can be a transistor obtained in such a manner that an element isolation insulating layer is provided on a substrate including a semiconductor material (e.g., silicon) and a region serving as the channel formation region is formed in a region surrounded by the element isolation insulating layer. Note that the transistor may be a transistor obtained in such a manner that the channel formation region is formed in a semiconductor film such as a polycrystalline silicon film formed on an insulating surface or in a silicon film of an SOI substrate.

The memory cell arrays 3400(1) to 3400(n) and the logic circuit 3004 are stacked with interlayer insulating layers provided therebetween, and electrical connection or the like may be established as appropriate by an electrode or a wiring penetrating the interlayer insulating layers.

When a transistor including an oxide semiconductor for a channel formation region and having extremely small off-state current is applied to the semiconductor device in this embodiment, the semiconductor device can store data for an extremely long period. In other words, power consumption can be adequately reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be held for a long period even when power is not supplied (note that a potential is preferably fixed).

Further, in the semiconductor device described in this embodiment, high voltage is not needed for writing data and there is no problem of deterioration of elements. For example, unlike a conventional non-volatile memory, it is not necessary to inject and extract electrons into and from a floating gate; thus, the problem of deterioration of a gate insulating film does not occur. In other words, the semiconductor device according to one embodiment of the present invention does not have a limit on the number of times of writing which is a problem in a conventional nonvolatile memory, and reliability thereof is drastically improved. Furthermore, data is written depending on the on state and the off state of the transistor, whereby high-speed operation can be easily realized.

As described above, a miniaturized and highly-integrated semiconductor device having high electrical characteristics and a method for manufacturing the semiconductor device can be provided.

This embodiment can be freely combined with any of Embodiment 1, Embodiment 2, and Embodiment 3.

Embodiment 5

In this embodiment, a central processing unit (CPU) at least part of which includes any of the transistors 440 a, 440 b, 440 c, 440 d, 441 a, 441 b, 441 c, 441 d, and 441 e described in Embodiment 1 or 2 is described as an example of a semiconductor device in this embodiment.

FIG. 11A is a block diagram illustrating a specific structure of the CPU. The CPU illustrated in FIG. 11A includes an arithmetic logic unit (ALU, an arithmetic logic circuit) 1191, an ALU controller 1192, an instruction decoder 1193, an interrupt controller 1194, a timing controller 1195, a register 1196, a register controller 1197, a bus interface (Bus I/F) 1198, a rewritable ROM 1199, and an ROM interface (ROM I/F) 1189 over a substrate 1190. A semiconductor substrate, an SOI substrate, a glass substrate, or the like is used as the substrate 1190. The ROM 1199 and the ROM interface 1189 may each be provided over a separate chip. Obviously, the CPU shown in FIG. 11A is just an example in which the configuration has been simplified, and an actual CPU may have various configurations depending on the application.

An instruction that is input to the CPU through the bus interface 1198 is input to the instruction decoder 1193 and decoded therein, and then, input to the ALU controller 1192, the interrupt controller 1194, the register controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the register controller 1197, and the timing controller 1195 conduct various controls in accordance with the decoded instruction. Specifically, the ALU controller 1192 generates signals for controlling the operation of the ALU 1191. While the CPU is executing a program, the interrupt controller 1194 judges an interrupt request from an external input/output device or a peripheral circuit on the basis of its priority or a mask state, and processes the request. The register controller 1197 generates an address of the register 1196, and reads/writes data from/to the register 1196 in accordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operation timings of the ALU 1191, the ALU controller 1192, the instruction decoder 1193, the interrupt controller 1194, and the register controller 1197. For example, the timing controller 1195 includes an internal clock generator for generating an internal clock signal CLK2 based on a reference clock signal CLK1, and supplies the internal clock signal CLK2 to the above circuits.

In the CPU illustrated in FIG. 11A, a memory cell is provided in the register 1196. As the memory cell of the register 1196, the memory cell described in Embodiment 4 can be used.

In the CPU illustrated in FIG. 11A, the register controller 1197 selects an operation of holding data in the register 1196 in accordance with an instruction from the ALU 1191. That is, the register controller 1197 selects whether data is held by a flip-flop or a capacitor in the memory cell included in the register 1196. When data holding by the flip-flop is selected, power supply voltage is supplied to the memory cell in the register 1196. When data holding by the capacitor is selected, the data is rewritten in the capacitor, and supply of power supply voltage to the memory cell in the register 1196 can be stopped.

The power supply can be stopped by providing a switching element between a memory cell group and a node to which a power supply potential VDD or a power supply potential VSS is supplied, as illustrated in FIG. 11B or FIG. 11C. Circuits illustrated in FIGS. 11B and 11C are described below.

FIGS. 11B and 11C each illustrate an example of a structure of a memory circuit in which any of the transistors 440 a, 440 b, 440 c, 440 d, 441 a, 441 b, 441 c, 441 d, and 441 e described in Embodiment 1 or 2 is used as a switching element for controlling supply of a power supply potential to a memory cell.

The memory device illustrated in FIG. 11B includes a switching element 1141 and a memory cell group 1143 including a plurality of memory cells 1142. Specifically, as each of the memory cells 1142, the memory cell described in Embodiment 3 can be used. Each of the memory cells 1142 included in the memory cell group 1143 is supplied with the high-level power supply potential VDD via the switching element 1141. Further, each of the memory cells 1142 included in the memory cell group 1143 is supplied with a potential of a signal IN and the low-level power supply potential VSS.

In FIG. 11B, any of the transistors 440 a, 440 b, 440 c, 440 d, 441 a, 441 b, 441 c, 441 d, and 441 e described in Embodiment 1 or 2 is used as the switching element 1141, and the switching of the transistor is controlled by a signal Sig A supplied to a gate electrode layer thereof.

Note that FIG. 11B illustrates the structure in which the switching element 1141 includes only one transistor; however, without particular limitation thereon, the switching element 1141 may include a plurality of transistors. In the case where the switching element 1141 includes a plurality of transistors which serves as switching elements, the plurality of transistors may be connected to each other in parallel, in series, or in combination of parallel connection and series connection.

Although the switching element 1141 controls the supply of the high-level power supply potential VDD to each of the memory cells 1142 included in the memory cell group 1143 in FIG. 11B, the switching element 1141 may control the supply of the low-level power supply potential VSS.

In FIG. 11C, an example of a memory device in which each of the memory cells 1142 included in the memory cell group 1143 is supplied with the low-level power supply potential VSS via the switching element 1141 is illustrated. The supply of the low-level power supply potential VSS to each of the memory cells 1142 included in the memory cell group 1143 can be controlled by the switching element 1141.

When a switching element is provided between a memory cell group and a node to which the power supply potential VDD or the power supply potential VSS is supplied, data can be held even in the case where an operation of a CPU is temporarily stopped and the supply of the power supply voltage is stopped; accordingly, power consumption can be reduced. Specifically, for example, while a user of a personal computer does not input data to an input device such as a keyboard, the operation of the CPU can be stopped, so that the power consumption can be reduced.

Although the CPU is given as an example, the transistor can also be applied to an LSI such as a digital signal processor (DSP), a custom LSI, or a field programmable gate array (FPGA).

The structures, methods, and the like which are described in this embodiment can be combined as appropriate with any of the structures, methods, and the like which are described in the other embodiments.

Embodiment 6

A semiconductor device disclosed in this specification can be applied to a variety of electronic devices (including game machines). Examples of the electronic devices include display devices of televisions, monitors, and the like, lighting devices, desktop personal computers and laptop personal computers, word processors, image reproduction devices which reproduce still images or moving images stored in recording media such as digital versatile discs (DVDs), portable compact disc (CD) players, radio receivers, tape recorders, headphone stereos, stereos, cordless phone handsets, transceivers, portable wireless devices, mobile phones, car phones, portable game machines, calculators, portable information terminals, electronic notebooks, e-book readers, electronic translators, audio input devices, cameras such as still cameras and video cameras, electric shavers, high-frequency heating appliances such as microwave ovens, electric rice cookers, electric washing machines, electric vacuum cleaners, air-conditioning systems such as air conditioners, dishwashers, dish dryers, clothes dryers, futon dryers, electric refrigerators, electric freezers, electric refrigerator-freezers, freezers for preserving DNA, smoke detectors, radiation counters, and medical equipment such as dialyzers. Further, the examples include industrial equipment such as guide lights, traffic lights, belt conveyors, elevators, escalators, industrial robots, and power storage systems. In addition, oil engines, moving objects driven by electric motors using power from the non-aqueous secondary batteries, and the like are also included in the category of electronic devices. Examples of the moving objects include electric vehicles (EV), hybrid electric vehicles (HEV) which include both an internal-combustion engine and a motor, plug-in hybrid electric vehicles (PHEV), tracked vehicles in which caterpillar tracks are substituted for wheels of these vehicles, motorized bicycles including motor-assisted bicycles, motorcycles, electric wheelchairs, golf carts, boats, ships, submarines, helicopters, aircrafts, rockets, artificial satellites, space probes, planetary probes, spacecrafts, and the like. Specific examples of these electronic devices are illustrated in FIGS. 12A to 12C and FIGS. 13A to 13C.

FIGS. 12A and 12B illustrate a tablet terminal that can be folded in two. FIG. 12A illustrates the tablet terminal which is open. The tablet terminal includes a housing 9630, a display portion 9631 a, a display portion 9631 b, a switch 9034 for switching display modes, a power switch 9035, a switch 9036 for switching to power-saving mode, a fastener 9033, and an operation switch 9038.

In such a portable device illustrated in FIGS. 12A and 12B, an SRAM or a DRAM is used as a memory element for temporarily storing image data. For example, the semiconductor device described in Embodiment 4 can be used as a memory. The semiconductor device described in the above embodiment employed for the memory element enables writing and reading of data to be performed at high speed, enables data to be retained for a long time, and enables power consumption to be sufficiently reduced. A CPU for performing image processing or arithmetic processing is used in the portable device illustrated in FIGS. 12A and 12B. As the CPU, the CPU described in Embodiment 5 can be used, in which case the CPU described in Embodiment 5 is used, power consumption of the portable device can be reduced.

Part of the display portion 9631 a can be a touch panel region 9632 a, and data can be input by touching operation keys 9638 that are displayed. Note that FIG. 12A shows, as an example, that half of the area of the display portion 9631 a has only a display function and the other half of the area has a touch panel function. However, the structure of the display portion 9631 a is not limited to this, and all the area of the display portion 9631 a may have a touch panel function. For example, all the area of the display portion 9631 a can display keyboard buttons and serve as a touch panel while the display portion 9631 b can be used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b can be a touch panel region 9632 b. When a finger, a stylus, or the like touches the place where a button 9639 for switching to keyboard display is displayed in the touch panel, keyboard buttons can be displayed on the display portion 9631 b.

Touch input can be performed concurrently on the touch panel regions 9632 a and 9632 b.

The switch 9034 for switching display modes can switch display orientation (e.g., between landscape mode and portrait mode) and select a display mode (switch between monochrome display and color display), for example. With the switch 9036 for switching to power-saving mode, the luminance of display can be optimized in accordance with the amount of external light at the time when the tablet terminal is in use, which is detected with an optical sensor incorporated in the tablet terminal. The tablet terminal may include another detection device such as a sensor for detecting orientation (e.g., a gyroscope or an acceleration sensor) in addition to the optical sensor.

Although FIG. 12A shows the example where the display area of the display portion 9631 a is the same as that of the display portion 9631 b, there is no particular limitation on the display portions 9631 a and 9631 b. They may differ in size and/or image quality. For example, one of them may be a display panel that can display higher-definition images than the other.

FIG. 12B illustrates the tablet terminal which is closed. The tablet terminal includes the housing 9630, a solar battery 9633, a charge/discharge control circuit 9634, a battery 9635, and a DC to DC converter 9636. As an example, FIG. 12B illustrates the charge/discharge control circuit 9634 including the battery 9635 and the DC to DC converter 9636.

Since the tablet terminal can be folded in two, the housing 9630 can be closed when the tablet terminal is not in use. Thus, the display portions 9631 a and 9631 b can be protected, thereby providing a tablet terminal with high endurance and high reliability for long-term use.

The tablet terminal illustrated in FIGS. 12A and 12B can also have a function of displaying various kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, the time, or the like on the display portion, a touch-input function of operating or editing data displayed on the display portion by touch input, a function of controlling processing by various kinds of software (programs), and the like.

Power can be supplied to a touch panel, a display portion, an image signal processor, or the like by the solar battery 9633 attached on a surface of the tablet terminal Note that the solar battery 9633 can be provided on one or both surfaces of the housing 9630, so that the battery 9635 can be charged efficiently. The use of a lithium ion battery as the battery 9635 brings an advantage such as reduction in size.

The structure and operation of the charge/discharge control circuit 9634 illustrated in FIG. 12B are described with reference to a block diagram in FIG. 12C. FIG. 12C illustrates the solar battery 9633, the battery 9635, the DC to DC converter 9636, a converter 9637, switches SW1 to SW3, and the display portion 9631. The battery 9635, the DC to DC converter 9636, the converter 9637, and the switches SW1 to SW3 correspond to the charge/discharge control circuit 9634 illustrated in FIG. 12B.

An example of the operation performed when power is generated by the solar battery 9633 using external light is described. The voltage of power generated by the solar battery is raised or lowered by the DC to DC converter 9636 so as to be a voltage for charging the battery 9635. Then, when power from the solar battery 9633 is used for the operation of the display portion 9631, the switch SW1 is turned on and the voltage of the power is raised or lowered by the converter 9637 so as to be a voltage needed for the display portion 9631. When images are not displayed on the display portion 9631, the switch SW1 is turned off and the switch SW2 is turned on so that the battery 9635 is charged.

Here, the solar battery 9633 is shown as an example of a power generation means; however, there is no particular limitation on a way of charging the battery 9635, and the battery 9635 may be charged with another power generation means such as a piezoelectric element or a thermoelectric conversion element (Peltier element). For example, the battery 9635 may be charged with a non-contact power transmission module that transmits and receives power wirelessly (without contact) to charge the battery or with a combination of other charging means.

In a television set 8000 in FIG. 13A, a display portion 8002 is incorporated in a housing 8001. The display portion 8002 displays an image and a speaker portion 8003 can output sound.

A semiconductor display device such as a liquid crystal display device, a light-emitting device in which a light-emitting element such as an organic EL element is provided in each pixel, an electrophoresis display device, a digital micromirror device (DMD), or a plasma display panel (PDP) can be used for the display portion 8002.

The television set 8000 may be provided with a receiver, a modem, and the like. A general television broadcast can be received with the receiver in television set 8000. Furthermore, when the television set 8000 is connected to a communication network by wired or wireless connection via the modem, one-way (from a transmitter to a receiver) or two-way (between a transmitter and a receiver, between receivers, or the like) data communication can be performed.

In addition, the television set 8000 may include a CPU for performing information communication or a memory. The memory described in Embodiment 4 or the CPU described in Embodiment 5 can be used in the television set 8000.

In FIG. 13A, an air conditioner which includes an indoor unit 8200 and an outdoor unit 8204 is an example of an electric device in which the CPU of Embodiment 5 is used. Specifically, the indoor unit 8200 includes a housing 8201, an air outlet 8202, a CPU 8203, and the like. Although the CPU 8203 is provided in the indoor unit 8200 in FIG. 13A, the CPU 8203 may be provided in the outdoor unit 8204. Alternatively, the CPU 8203 may be provided in both the indoor unit 8200 and the outdoor unit 8204. By using the CPU described in Embodiment 5 as the CPU in the air conditioner, power consumption can be reduced.

In FIG. 13A, an electric refrigerator-freezer 8300 is an example of an electric device which is provided with the CPU formed using an oxide semiconductor. Specifically, the electric refrigerator-freezer 8300 includes a housing 8301, a door for a refrigerator 8302, a door for a freezer 8303, a CPU 8304, and the like. The CPU 8304 is provided in the housing 8301 in FIG. 13A. When the CPU described in Embodiment 5 is used as the CPU 8304 of the electric refrigerator-freezer 8300, power consumption of the electric refrigerator-freezer 8300 can be reduced.

FIG. 13B illustrates an example of an electric vehicle. An electric vehicle 9700 is equipped with a secondary battery 9701. The output of the electric power of the secondary battery 9701 is adjusted by a control circuit 9702 so that the electric power is supplied to a driving device 9703. The control circuit 9702 is controlled by a processing unit 9704 including a ROM, a RAM, a CPU, or the like which is not illustrated. By using the CPU described in Embodiment 5 as the CPU in the electric vehicle 9700, power consumption can be reduced.

The driving device 9703 includes a DC motor or an AC motor either alone or in combination with an internal-combustion engine. The processing unit 9704 outputs a control signal to the control circuit 9702 based on input data such as data of operation (e.g., acceleration, deceleration, or stop) by a driver or data during driving (e.g., data on an upgrade or a downgrade, or data on a load on a driving wheel) of the electric vehicle 9700. The control circuit 9702 adjusts the electric energy supplied from the secondary battery 9701 in accordance with the control signal of the processing unit 9704 to control the output of the driving device 9703. In the case where the AC motor is mounted, although not illustrated, an inverter which converts direct current into alternate current is also incorporated.

This embodiment can be implemented combining with another embodiment as appropriate.

Embodiment 7

In Embodiments 3 and 4, the example in which the insulating film 434 is provided over the conductive layer 491 and openings for exposing the top surfaces of the wiring layers 692, 834, and 835 are provided is described. In this embodiment, an example in which the top surfaces of the wiring layers 834 and 835 are exposed by planarization treatment such as CMP in order to reduce the number of steps is illustrated in FIG. 14.

In this embodiment, the same reference numerals are used for the same parts as those in Embodiment 4.

In the semiconductor device illustrated in FIG. 14, a single crystal silicon substrate is used as the substrate 800, the transistor 802 is formed using the single crystal silicon substrate, a transistor 815 in which stacked oxide semiconductor films are used for a channel formation region is formed over the transistor 802.

A gate electrode layer 401 of the transistor 815 is electrically connected to the wiring layer 832. The wiring layer 832 is electrically connected to the wiring layer 835. The gate electrode layer 401 of the transistor 815 is electrically connected to the embedded wiring, and the embedded wiring is electrically connected to the conductive layer 842. Note that the embedded wiring includes the first barrier metal film 486, the second barrier metal film 488, and the low-resistance conductive layer 487 surrounded by the first barrier metal film 486 and the second barrier metal film 488.

The wiring layer 832 is formed over the insulating films 826 and 830, and the wiring layer 835 is formed in an opening in the insulating film 833.

The electrode layer 825 of the transistor 802 is electrically connected to the electrode layer 845 b of the transistor 803 through the wiring layers 831 and 834. The wiring layer 831 is formed in an opening in the insulating film 830, and the wiring layer 834 is formed in an opening in the insulating film 833 and electrically connected to the wiring layer 831. The wiring layers 834 and 835, a wiring layer 836, and the conductive layer 491 are formed over the insulating film 833, and the oxide insulating film 435 is formed. After that, the oxide insulating film 435 is planarized by CMP treatment, so that the top surfaces of the wiring layers 834, 835, and 836 and the conductive layer 491 are exposed.

After the planarization, the insulating film 437 and the oxide semiconductor film 403 are formed and selectively etched by patterning with the use of one mask. The oxide semiconductor film 403 may be stacked oxide semiconductor films having different compositions.

After the insulating film 437 and the oxide semiconductor film 403 are formed, a conductive film is formed. This conductive film is selectively etched, so that the electrode layers 845 a and 845 b and the conductive layer 842 are formed. By performing etching plural times at this time, electrodes each including projecting portions when seen in cross-section are formed. The electrode layers 845 a and 845 b each including projecting portions function as source and drain electrode layers of the transistor 815. The electrode layer 845 a is provided over and in contact with the wiring layer 836. The electrode layer 845 b is provided over and in contact with the wiring layer 834. The conductive layer 842 is provided over and in contact with the wiring layer 835.

The gate insulating film 402, which is a gallium oxide film, is provided over the electrode layers 845 a and 845 b and the conductive layer 842, and is over and in contact with the oxide semiconductor film 403. The gate electrode layer 401 is provided over the oxide semiconductor film 403 with the gate insulating film provided therebetween. The insulating film 407 functioning as a barrier film is provided so as to cover the gate electrode layer 401.

The transistor 815 can be used instead of the transistor 803 in FIGS. 8A and 8B so that the NOR circuit in FIG. 8B can be formed.

The number of steps for manufacturing the transistor 815 is smaller than that of the transistor 803; thus, manufacturing cost can be reduced.

This embodiment can be freely combined with any of the other embodiments.

Example 1

In this example, STEM images of the shapes of end portions of a top-gate transistor in FIG. 15A were observed in the middle of a process for manufacturing the transistor.

The top-gate transistor in FIG. 15A includes the oxide insulating film 435 over the substrate 400 which is formed of quartz glass, the first oxide semiconductor film 403 a, the second oxide semiconductor film 403 b, the gate insulating film 402 over the second oxide semiconductor film 403 b, the gate electrode layer 401 overlapping with the second oxide semiconductor film 403 b with the gate insulating film 402 positioned therebetween, and the insulating film 407 covering the gate electrode layer 401.

As the first oxide semiconductor film 403 a, a 5-nm-thick In—Ga—Zn-based oxide film formed with a target having an atomic ratio of In:Ga:Zn=3:1:2 was used. As the second oxide semiconductor film 403 b, a 10-nm-thick In—Ga—Zn-based oxide film formed with a target having an atomic ratio of In:Ga:Zn=1:1:1 was used.

After the second oxide semiconductor film 403 b was formed, a 100-nm-thick conductive film which was a single tungsten layer was formed. This conductive film was selectively etched, so that the electrode layers 845 a and 845 b were formed. By performing this etching, electrodes each including projecting portions when seen in cross-section were formed.

The conductive film was etched by dry etching using an inductively coupled plasma (ICP) etching method.

First etching conditions were as follows: an etching gas of Cl₂, CF₄, and O₂ (Cl₂:CF₄:O₂=45 sccm:45 sccm:55 sccm), a power of a power supply of 3000 W, a bias power of 110 W, and a pressure of 0.67 Pa. Second etching conditions were as follows: an etching gas of O₂ (O₂=100 sccm), a power of the power supply of 2000 W, a bias power of 0 W, and a pressure of 3 Pa. Third etching conditions were as follows: an etching gas of Cl₂, CF₄, and O₂ (Cl₂:CF₄:O₂=45 sccm:45 sccm:55 sccm), a power of the power supply of 3000 W, a bias power of 110 W, and a pressure of 0.67 Pa. The substrate temperature in etching was 40° C. in all the conditions.

A 20-nm-thick silicon oxynitride film was formed by a plasma CVD method as the gate insulating film 402. A cross-sectional view at this stage is shown in FIG. 15B.

FIG. 15C shows a cross-sectional STEM image of a region surrounded by dotted lines in FIG. 15B. As shown in FIG. 15C, a first taper angle θ1 was approximately 58° and a second taper angle θ2 was approximately 70°. The first taper angle θ1 is an angle formed by a planar surface of the substrate 400 and a first side surface of an end portion of the electrode layer 845 b as illustrated in FIG. 15B. The second taper angle θ2 is an angle formed by the planar surface of the substrate 400 and a second side surface of the end portion of the electrode layer 845 b as illustrated in FIG. 15B.

Then, etching conditions were changed from the above-described etching conditions (i.e., the etching conditions were changed twice) to the following etching conditions. First etching conditions were as follows: an etching gas of Cl₂, CF₄, and O₂ (Cl₂:CF₄:O₂=45 sccm:45 sccm:55 sccm), a power of a power supply of 3000 W, a bias power of 110 W, and a pressure of 0.67 Pa. Second etching conditions were as follows: an etching gas of O₂ (O₂=100 sccm), a power of the power supply of 2000 W, a bias power of 0 W, and a pressure of 3 Pa. Third etching conditions were as follows: an etching gas of Cl₂, CF₄, and O₂ (Cl₂:CF₄:O₂=45 sccm:45 sccm: 55 sccm), a power of the power supply of 3000 W, a bias power of 110 W, and a pressure of 0.67 Pa. Fourth etching conditions were as follows: an etching gas of O₂ (O₂=100 sccm), a power of the power supply of 2000 W, a bias power of 0 W, and a pressure of 3 Pa. Fifth etching conditions were as follows: an etching gas of Cl₂, CF₄, and O₂ (Cl₂:CF₄: O₂=45 sccm:45 sccm:55 sccm), a power of the power supply of 3000 W, a bias power of 110 W, and a pressure of 0.67 Pa. The substrate temperature in etching was 40° C. in all of the conditions.

After the etching in which etching conditions were changed four times, as shown in FIG. 16A, the first taper angle θ1 of an end portion of the electrode layer 845 b was approximately 30° and the second taper angle θ2 of the end portion of the electrode layer 845 b was approximately 60°. An end portion of the electrode layer 845 a shown in FIG. 16B had a similar shape.

By adjusting etching conditions as appropriate in such a manner, the first taper angle θ1 and the second taper angle θ2 can be adjusted. The first taper angle θ1 is preferably 20° or greater and 70° or smaller, further preferably 25° or greater and 45° or smaller. In the case where the gate insulating film 402 has a small thickness of 20 nm, the first taper angle θ1 is preferably made 25° or greater and 45° or smaller, in which case coverage is improved.

This embodiment can be freely combined with any of the other embodiments.

This application is based on Japanese Patent Application serial No. 2012-096952 filed with Japan Patent Office on Apr. 20, 2012 and Japanese Patent Application serial No. 2012-118815 filed with Japan Patent Office on May 24, 2012, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: a first conductive layer; a first insulating film over the first conductive layer; an oxide semiconductor film which is over the first insulating film and comprises a channel formation region; a source electrode layer and a drain electrode layer which are over the oxide semiconductor film and are electrically connected to the oxide semiconductor film; a second insulating film over the oxide semiconductor film, the source electrode layer, and the drain electrode layer; and a second conductive layer which is over the second insulating film and overlaps with the channel formation region, wherein the source electrode layer and the drain electrode layer are single-layer metal films and each include projecting portions extending in the channel length direction, and wherein some of the projecting portions which overlap with the oxide semiconductor film overlap with the first conductive layer.
 2. The semiconductor device according to claim 1, wherein the first insulating film comprises an oxygen-excess region, and wherein the first insulating film has a smaller thickness in a region overlapping with the first conductive layer than in another region.
 3. The semiconductor device according to claim 1, wherein the oxide semiconductor film is stacked oxide semiconductor films having different compositions.
 4. The semiconductor device according to claim 1, wherein the oxide semiconductor film comprises a depressed portion and overlaps with the first conductive layer, and wherein a region having a small thickness of the oxide semiconductor film is the channel formation region.
 5. The semiconductor device according to claim 1, wherein a channel length is a distance between a projecting portion which is one of the projecting portions of the source electrode layer extending in the channel length direction and overlaps with the first conductive layer and the oxide semiconductor film and a projecting portion which is one of the projecting portions of the drain electrode layer extending in the channel length direction and overlaps with the first conductive layer and the oxide semiconductor film.
 6. The semiconductor device according to claim 1, further comprising: a third insulating film overlapping with the source electrode layer and the drain electrode layer, wherein the second insulating film is over and in contact with the first insulating film.
 7. A semiconductor device comprises: a first conductive layer; a first insulating film over the first conductive layer; a second insulating film containing gallium oxide over the first insulating film; an oxide semiconductor film which is over and in contact with the second insulating film and comprises a channel formation region; a source electrode layer and a drain electrode layer which are over the oxide semiconductor film and are electrically connected to the oxide semiconductor film; a third insulating film containing gallium oxide over the oxide semiconductor film, the source electrode layer, and the drain electrode layer; a second conductive layer which is over the third insulating film and overlaps with the channel formation region, wherein the source electrode layer and the drain electrode layer are single-layer metal films and each include projecting portions extending in the channel length direction, and wherein some of the projecting portions which overlap with the oxide semiconductor film overlap with the first conductive layer.
 8. The semiconductor device according to claim 7, wherein the oxide semiconductor film has a crystal structure, and wherein the second insulating film and the third insulating film have a lower crystallinity than the oxide semiconductor film.
 9. The semiconductor device according to claim 7, wherein the second insulating film has the same top shape as that of the oxide semiconductor film including the channel formation region.
 10. The semiconductor device according to claim 7, wherein the oxide semiconductor film is stacked oxide semiconductor films having different compositions.
 11. The semiconductor device according to claim 7, wherein the oxide semiconductor film comprises a depressed portion and overlaps with the first conductive layer, and wherein a region having a small thickness of the oxide semiconductor film is the channel formation region.
 12. The semiconductor device according to claim 7, wherein a channel length is a distance between a projecting portion which is one of the projecting portions of the source electrode layer extending in the channel length direction and overlaps with the first conductive layer and a projecting portion which is one of the projecting portions of the drain electrode layer extending in the channel length direction and overlaps with the first conductive layer.
 13. The semiconductor device according to claim 7, further comprising: a fourth insulating film overlapping with the source electrode layer and the drain electrode layer, wherein the third insulating film is over and in contact with the first insulating film. 